Every match-play world champion reached Olympus, so we're comparing the gods of the chess world. But for me, Petrosian was a considerably more accomplished world champion than Spassky.
Petrosian defeated Botvinnik in convincing fashion. He made the Patriarch look a bit like a novice in terms of positional understanding. Then Petrosian defended his title against Spassky before losing the rematch. Petrosian remained one of the most powerful chess players in the world in the 1970s, and arguably had a better result against Fischer in the semis than Spassky did in the finals in 1972.
Petrosian won the title and defended it successfully. He played famous and difficult matches in the 1970s against Fischer and Korchnoi. In 1975, he won the USSR championship ahead of the likes of Tal, Bronstein, Geller, Polugaevsky, and Vaganian (who, at that time, was quite a bit more dangerous than Bronstein). In 1977, Korchnoi barely beat Petrosian 6.5-5.5. Even as late as 1981, Petrosian beat Kasparov as Black in a Queen's Gambit Accepted.
Petrosian had plus scores against Euwe, Botvinnik, Bronstein, Smyslov, Tal and Karpov. He had even records against Spassky, Korchnoi and Kasparov. The only world champion or world champion candidate of his generation he had a losing record to was Fischer.
Petrosian never had quite the same accomplishments as Botvinnik, Smyslov or Tal, but his successes were greater than Spassky's. And his legacy as the greatest defender in chess history gives him a unique place in the pantheon of chess greats.
Nice article. It is quaint and packed full of legitimate, powerful information. I'll give it a 9. I'd give it a 10 if you laid in on thicker... lol 
Petrosian is one of my favourite players of all time along with Spassky, and Lasker. He was virtually unbeatable and just because he was a very positional accurate player does not mean he was unable to play tactically or create combinations. There is nothing wrong at all with playing very sound positional chess and exploiting very small advantages of your opponent when that opponent can't play such sound accurate chess as you can. What that means is you are the better chess player. You know a chess player does not become World Champion unless they are a brilliant chess player. Incidentally, take a look at my user name and make an anagram of the letters and what do you get?