The past few games on this have been a little frustrating, partially because I can't work out where we are going wrong. Even when we have good games as a group we still seem to lose so I have opted to have a break from it the past few evenings.
As for my go to characters, they differ from the suggestions above and that's because they much more suit how I play -
Tank - My go to tank is Peni everytime, she is such an aggressive tank and if you have a good healer behind you, you can really wreck teams. I also have a few clips where I have used the Ult and held off or even killed the whole opposition team after the rest of my side has been taken out. You just sometimes have to be clever about where you position your auto mine device.
DPS - I seem to be struggling a little bit with DPS at the moment. I had a good game with Scarlett Witch the other night, the first time she has really clicked for me but as good as her attack is the range is quite limited which means you have to keep darting in and out and timing it so you don't suddenly get pulverized as she is a little squishy. I enjoy playing squirrel girl as her explosive shots can cause some damage even from some distance away though I don't think I am getting the best out of her yet. Then playing as Mr Fantastic is like having an extra tank which is absurd. It's great for holding a point a little while longer after your team has been killed off but he is so frustrating to play against.
Healer - I think Cloak and dagger is great, being able to switch to attack when required too otherwise Luna Snow is probably my other choice. Her speed can help you get into the fray as and when required but healing is quite easy to play with as well.