I just wanted to make a mention of which hero is my current favorite infiltrator on MAA iOS for PvE. (This review is only among the heros that I have recruited. I do not have any alt uniforms.)
Hand's down, my pick is Psylocke for any situation that requires an infiltrator.
Her Psychic Scanning passive is currently broken on iOS and does zero damage, but that bug doesn't detract from Psylocke's usefulness. (I believe that the problem is that Psychic Scanning triggers currently triggers 100% of the time and crits almost at 100%, so that's why the damage is currently set to zero.)
Mental Coordination is a fantastic buff. I only wish it triggered more often, but when it does it devastates the enemy. It may seem like a waste of a turn to use this buff, but it also activates Psionic Shadow which makes her slightly harder to hit, and the potential chance of additional attacks from Mental Coordination plus increased stats on all allies is worth it.
Her two attacks deal fairly good damage and hits fairly consistently and she has the ability to cast a shield on an ally on top of it all. Because of all these reasons, Psylocke can be useful in any situation.
Great damage, ability to remove enemy buffs even if her L9 power is her passive skill. Because Black Widow is almost completely offensive in nature, she's not as versatile.
Other Infiltrators' Weaknesses:
Invisible Woman would probably as good a choice as Psylocke with her team shield and AoE attack, but instead of having the Force Cage power as other platforms have, iOS get's her passive skill as her L2 ability. If Invisible Woman had Force Cage, then she would probably be tied with Psylocke for best all-around infiltrator, but without it, there's little reason to choose Invisible Woman if Psylocke has been recruited.
Kitty Pryde's Phased ability does not last enough rounds and Lockheed does not make appearances often enough which makes her far too vulnerable. Since her damage output is not that good and she's still far too easy to hit, she's not even a good choice as a tank for the team.
Spider-Man should be awesome, but even fully socketed with Speedy ISO still allows him to get hit far too often. His damage output isn't great, which means Spider-Man gets knocked out far too soon.
Tigra and Black Cat are both very nice offensive heroes that both inflict bleed on enemies. Black Cat needs to first chain Stroke of Luck to really deal good damage, which only gives you a good strike in the second turn. Tigra's Hunter's Senses passive does not start reducing damage taken until after she starts inflicting bleed on enemies. As such, her passive skill doesn't effectively protect her from damage.