Who is the "best" teamup character? I was gone when this mechanic was introduced and its all new to me. Playing mainly Ms. Marvel.
A brief overview of the 4:
There are 3 modes you can set teamups as: Passive, Always out, 30 second mode.
For the most part, always out mode is pretty crappy unless you're leveling up or in very rare situations where you can keep protect them. Their damage is fairly bad, their AI is terrible (waste 1 minute cooldown attacks on the wrong targets, etc), and they're squishy even with regen.
30 second mode, the only viable hero for this is Firestar. The others just have way too low damage output to even consider in this mode. It's a net loss for practically anything other than cosmic terminal farming (kill to boss, cast it, kill boss, switch characters, do it again).
Passively, it depends on your hero. Magik gives 52 life steal a hit and 10% summon ally damage, Firestar gives 5% spirit reduction, Spiderman gives 700ish dodge and a damage proc on dodging, and Falcon gives crit rating.
Spiderman has the best passive 99% of the time, but especially on low dodge heroes as it suffers the least amount of diminishing returns. He also has 2 really good passive damage procs on top of that. He's probably the best hero to use passively.
Magik's 52 life steal isn't great but can be stacked with other sources to make it "not bad". She comes with 1 good damage proc and 1 crappy summon skill passively though.
Firestar's passive is probably the worst except for those who SERIOUSLY need the spirit reduction. She does come with 2 "okay" damage procs which would beat out Magik.
Falcon sucks. The crit rating can be found on teamup gear now just further diminishes it's value. If you're seriously lacking crit, you can pick him though. His attack speed proc suffers diminishing returns and doesn't benefit all builds either, leaving him with just 1 damage proc.
All damage procs scale based off YOUR damage rating, and type does matter. So physical-damage based heroes (Thing/Spiderman) will deal more damage with Spiderman/Falcon passive procs than a Scarlet Witch using those characters. This diminishes Magik's value somewhat because there are less mental damage heroes and the procs become way worse when used on a non-mental damage hero.
So to sum up; "best" teamup is likely Firestar/Spiderman. They both have their uses. Firestar can work either in all 3 modes reasonably well, spiderman only works in 2 but is probably stronger passively.
Magik isn't bad, but more specialized. Falcon sucks, I don't use him on anyone
