... drum roll...
1) Gambit 1) Psylocke
2) Wolverine 2) Black Widow
3) Tigra 3) Shadowcat
4) Wasp 4) Spider-Man
5) Phoenix 5) Black Cat
6) Mr. Fantastic 6) Invisible Woman
7) Hawkeye 7) Nightcrawler
To make it completely clear, no character can take as many hits as Kitty. Boost her Evasion and Defense all you like, but using her well is all about practice and timing. If you want to do something useful with her Iso's, two words: Deadly Crits, which have allowed me to one-hit Captain America on rare occassion. The Achilles' heel in her defense? The game-changing Magik, and don't get me started on the AvX Set!
As good as Kitty is, she doesn't get my vote for best Infiltrator, nor do Gambit or Black Cat. Both are hard hitters, but rely on build-up to do major damage. For a hero that roles of lots of damage right away, Black Widow makes both lists for the best reason. But the next best reason is also a damn good one. As good as Black Widow is with a class advantage (Infiltrator vs. Tactican or Tactician vs. Blaster), she works just as hard in a counter-class battle (Tactician vs. Infilitrator or Infiltrator vs. Scrapper). Level her up to 9 for Revenge Tactics, and if you were smart enough to grab either (or both) variants of her Assemble suits, team her with a fellow Avenger (Cap, Thor, or Hawkeye work best) or an Agent rocking Coulsons Revenge, and her defensive prowess comes into its own. This class is all about versatility, and for what my vote is worth, she is the most versatile of them all. If i slighted anyone's favorite character, argue for them in the comments! What makes them pay off for you?
Advice: A special word on Spider-Man, Hawkeye, and Phoenix. These three characters share the special quality of having costumes in three classes, whereas most only have two. Hawkeye is pretty straight-forward, if you got the Avengers costumes while they were available, good thinking, because both his and Widow's make them lethal. If you playstyle is the total opposite of mine, both Hawkeye and Phoenix come in Blaster, and you could easily max out your attack the same way I've maxed my Evade.
P5 Phoenix is a vast improvement, but she still needs extra protection. For me, that's all the more reason to avoid the Blaster costume, unless your strategy in to hit hit so hard and so fast that defense doesn't matter. Best of luck.
Spider-Man is a little bit trickier, as each of his three alternates possesses something unique. Evasion is his strongest trait and the FF Infiltrator plays to that. Scrapper Spidey has the potential for lots of damage, but getting nailed by an double-Enraged Bruiser will make short work of him. The new Close Quarters buff makes this that much easier to do. Finally, his Black Costume offers the most interesting possibilities: trading Great Responsibility for Great Power at the beginning of the match. Packing plenty of Strong Iso's onto him, and this Spider-Man hits hard and doesn't slavishly offer himself up for punishment.