Spassky was a stud. Unfortunately, he's best known for losing to Fischer....but anyone in that day and age would've gotten their butt kicked by Bobby.
He was a gutsy player....willing to play the King's Gambit. He also added a lot to the theory behind the closed Sicilian.
He tends to be associated with lackluster performances vs. Fischer and relegated to a lower tier of chess player, perhaps not even great. Yet in his run up to finally gaining the title in 1969 he displayed a virtuosity in a variety of styles selected to defeat particular opponents, namely Keres, Geller (twice), Tal, Larsen, and Korchnoi. He lost one match to Petrosian (1966) and won in 1969 for the WC title.
His failing was that he was not monomaniacal about chess like Tal and Fischer, enjoying life as a well-rounded character. Similarly, Capablanca couldn't be persuaded to work like Alekhine, and he lost to him. It was predicted that he would have his title for one cycle and no more.
That is what happened.