I've given a few of each a go!
DPS - I've tried a few, Black widow isn't bad as I do enjoy sniping and seem to be quite accurate, bucky I'm slowly trying to get the hang of, same with wolverine, however whilst playing ranked I've settled on Punisher, I think people massively underestimate his smoke bomb and zipwire, yes his guns are class but when team mates die and your up against it maybe 2 v 6 for a limited time, smokescreen then zipwhire to get the hell back to wait for your team. Also linking up with rocket or a good healer who gives you increased damage or bullets and your near unbeatable!
Tank - Sorry but I will only stick with one on this and that is Groot, the guy is a legit cheat code, using your larger walls to help block damage, staying close to them helps heal you, then using your smaller walls, try if you can to put them above your enemies so they can't see and won't shoot them down, so then when activated will deal out some decent damage whilst using combination of R2 and right analog, his grenade bomb is pretty good also, going back to the walls, they can really help get you those extra seconds needed on the point, blocking off certain entry points for the enemy. His Ult is perfect for setting up a team mate who can deal some damage, grabs multiple enemies at a time and delivers some great damage, keeping them still for around 5 seconds which is more than enough time for someone to dish out some destruction. To sum up....GROOT IS KING!
Healer - I have tried using Jeff and if used correctly can be an irritating little shit to the opposite team, especially if they are all occupying the point, his ult can take the whole team if needed and launch them clean off the map, his healing process via bubbles and water gun are good but you have to be accurate with that. My go to for healer though is Adam Warlock, I don't he's best being used as a more defensive healer, keeping out of site if possible, don't get me wrong, he can deal damage, if you build up his L2 beam and nail it, easily capable of nearly eliminating someone, combine that with his regular R2 move and it's pretty decent combo, his healing is fantastic though, constantly being able to pick which team-mate to heal via R1 then also using L1 to share the damage throughout your whole team which if used properly is a God send, his ult if timed right is perfection, I think if you use it within 5/10 seconds of team mates dying, can all be revived instantly!
So yeah my go to guys are
DPS - Punisher
Tank - Groot
Healer - Adam Warlock
Favourite Player - Groot!