Gem Grab- Gene: Can be a gem carrier and support as well as grab their gem carrier
Solo Showdown- 8-Bit: That star power though, still good without it
Duo Showdown- Poco: Can heal teammates and himself when in danger and relies on teammate destroy boxes
Graveyard Shift- Piper: One hit insta heal to max
Brawl Ball- Jessie: Just really good in general. Throw turret in a good position and has the same range as Colt.
Siege- Tick: Jesus is he annoying in this gamemode and he is good
Heist-Dynamike: Mediocre health but is a high damaging thrower
Bounty-Piper/Brock: Long range high damage
Takedown-Colt: Better health than Rico and does high damage per Attack at Long range
Lone Star- Bo: That Star Power though, good on all maps including island invasion
Boss Fight-Colt-Same reason as takedown
Robo Rumble-Pam: Heals with Super and high dps close range
Big Game-Jessie: That bounce though, both star powers good but Shocks preferred
If you disagree with any of these please say so in the comments but this is my opinion, not yours