What about being outclassed by a worse player?
Which loss is harder: being outclassed or losing to a worse player?
Hmm...But then how would you ascertain such a player was indeed "worse"? He/she did just outclass you after all...

A person who still has a provisional rating. Underestimating because you see the low rating and think "Oh, this is gonna be a cake walk."

You can't assume your opponent is 'worse'. Respect all opponents and fear none. If you believe you're the stronger player, you've got to prove it by winning.
I don't mind being genuinely outplayed - I'm not the strongest player in the world at the end of the day, and someone beats me because they just played better than that's something to learn from. I get annoyed if I beat myself, e.g. make an error for no apparent reason, regardless of who the opponent is.

I think it's probably a good idea to try to learn from your losses regardless of what the rating of the other player is. If you analyze the game, and pinpoint where you went wrong, you'll be ready next time.

Just like Wayne, I always try to draw lessons from my defeats, and also to find something positive to take with me from them. (Maybe I played a strong or interesting sacrifice, or maybe I defended tenaciously after an early mistake, etc.) That said, some pills are obviously more bitter to swallow than others. For me there are a couple:
1. Being completely outplayed positionally (totally different from being outcalculated in a tactical melée, which can happen anytime to anyone). This is a tricky one, because in some cases you may not even be aware or understand what happened, neither during nor after the game. You're only left with a vague, but persistent feeling of annoyance, and it can take a long time before you realize that your understanding of the position was utterly flawed. (Jacob Aagaard discusses this in one of his books, he happened to sit in on the post-mortem of a friend of his who had won a game against a lower-rated player, who lacked all understanding of the position they had ended up in. He kept suggesting new moves to save his game, only to be demolished time and again. He just couldn't fathom that his loss wasn't due to an individual blunder, but a positional understanding far inferior to that of his opponent.)
2. Losing because of your opening/variation choice. I'm not talking about an individual game where your opponent just happens to be better prepared on the day or know the latest wrinkle which you have missed (such as may happen in the Dragon Sicilian or the Semi-Slav Botvinnik variation). I'm talking about losses which occur because you persist in playing a variation which you're not quite able to master or perhaps is (seriously) unsound. You feel you've invested so much time that you don't want it to go to waste, and therefore you persist. And lose. And lose. And lose. For me, this happened as Black against the English. For years I played the Flohr-Mikenas variation (1.c4 Nf6 2.Nc3 e6 3.e4 c5 4.e5 Ng8), which requires very careful handling by Black, especially if you're playing strong opposition. My results were dismal.

For me losing against a weaker player because of my blunders is much worse because I didn't play well. There is nothing wrong with being outclassed by a stronger player. I am aware that there are many of them...

Sure, Jenium, there's absolutely no shame in losing against a stronger opponent - the rating should give you advance warning of this being a distinct possibility, so to speak. However, for me, losing because I haven't understood the position is not primarily a question of the opponent's superiority as such. It's more a case of chagrin at finding a gaping hole in my chess erudition, a hole I didn't even know I had. Does that make sense to you?

Drawing against stronger opponents is a de facto victory. Enjoy it when it happens.
On balance, you probably can learn more from your losses than your wins. So study the former, more often. It's a simple way to improve.
On balance, the games you win are usually because your opponent makes a mistake, and you gain material as a result. Typically there's less to learn from that scenario.
Everyone hates to lose. Regroup, rest, and get em' in the next round.

All losses are painful because after the fact I can see where I made a mistake. But losing to a significantly lower rated guy is worse for me. If I play a guy much stronger I treat it as a free hit, I can try whatever because in all likelehood he will beat me anyway.
Losing to a worse player is usually worse because you don't learn anything except that you need to humble yourself and be less cocky. But if you lose to a better player you can looking over the game and learn alot more. But all in all the only time losing to a worse player is bad for you is if they are disrespectful about it. Losing happens I lose all the time it's good for the morals if you can stop it from getting to you.

Neither the worse loss for me is when I kept playing an opening I know nothing about, losing and winning in an endless ebb and flow like the waves of the ocean, only to find myself starting from the beginning, like some piece of wood tossed by the waves and landed back on the shore.

Losing to a worse player is usually worse because you don't learn anything except that you need to humble yourself and be less cocky. But if you lose to a better player you can looking over the game and learn alot more. But all in all the only time losing to a worse player is bad for you is if they are disrespectful about it. Losing happens I lose all the time it's good for the morals if you can stop it from getting to you.
Don't you think this is slightly counterproductive? There may be many different reasons for your loss against an inferior opponent, some psychological, others purely chess-related.
If we look at a very high level, Kasparov was still much stronger generally than Kramnik when he lost the World Championship match in 2000. However, it was also clear that Kramnik at that point was a better player than Kasparov in quiet positions, especially without queens, with very little obvious going on on the surface. Equally, the Kasparov-Karpov rivalry over five matches and 144 games saw them separated by only 2 game points in the end. In tournaments, on the other hand, Kasparov regularly outperformed both Karpov and Kramnik by a considerable margin.
My point is that even if you lose a game against an inferior opponent, it could be either for psychological reasons, such as that you feel a lot of pressure to win or that your underestimate weaker opponents, and therefore underperform against them. If so, you need to find a way of resolving this issue. However, it could also be that your opponents has laid bare a weakness in your own knowledge or your approach to the game - after all, we're not complete aces! I used to have this problem against weaker opposition (but also against stronger players, but less noticeable against them for obvious reasons) that I got a good position, and then I started looking for a killer blow. Not finding any, I got into time pressure and squandered my advantage. It took quite some time for me to learn how to deal with these situations. One of the greatest clues to solving the problem was something Bent Larsen often used to write in his game annotations: 'It's not necessary to be completely winning, the important thing is to retain winning chances.' Put another way: keep making moves maintaining the pressure/advantage, and often your opponent will crack sooner or later.
Losing by spoiling a winning or drawn position with a mistake definitely hurts. You can play brilliantly for 30, 40, 50 moves, and then with one bad move throws it all away. That really leaves a mark.
But being outplayed by someone who simply plays better, esp. if they play a brilliant combination or something, feels OK. All you can do is admire their skill, enjoy the artistic aspect of what they created, and learn from the game.
Here is a tournament report I wrote a couple years ago, including analysis of my loss against IM Shmelov, where he outplayed me and finished with a very pretty combination.
http://www.masschess.org/chess_horizons/chess-horizons-article.aspx?ch_uid=228
Back when I allowed losses to really bother me, the hardest loss to deal with is the one in which you are completely outclassed OTB. Some people are the opposite - they see the loss as another benchmark; something to study and learn from - a mark to which they aspire in a quest to get better and learn. It took me a very long time to have this attitude, a journey even more challenging than the actual aspect of playing chess. If I was better but was careless and blunder - okay my opponent was lucky. No big deal. "Good game, ya got me. Maybe next time". But to be completely outclassed and beaten soundly by someone who sees more, knows more, and is clearly smarter...and better....that was very hard for me. It was all I could do to just say "good game" than all I want to do is find a hole and crawl into it. Complete disillusionment with chess. For me - it was an issue with pride and childishness. As I got older that slowly faded.. But sometimes it still tries to rear it's head but I won't let it..
What about YOU? How do you deal with loss? Which losses are the hardest for you to overcome?