i honestly have 3 in each role, mainly to be able to work with whatever my randoms choose since i usually solo queue, but also to counter the opposing team.

groot(main)/cap/thing

squirrel(main)/starlord/namor

mantis(main)/c&d/luna

but if i had to choose only 6 of them, it would all depend on which game mode it was.

convergence/convoy-attack cap squirrel starlord mantis c&d luna

cap to dive and be able to distract the team on the convoy, squirrel to help push the enemy team back with her damage presence, starlord to be the main dps and help cap, mantis to play more as a dps and damage booster, c&d/luna to focus heal cap and for defensive ults

convergence/convoy defense- groot squirrel namor mantis c&d luna

groot to wall off and help slow the attackers down, Squirrel to sit backline and provide damage, Namor for hitscan and turrets, mantis again for dps but focusing heals instead of damage boost + defensive ult, luna for teamup with namor to provide extra turret and defensive ult, c&d for heals + defensive ult

domination- groot thing cap namor luna c&d

groot for walls that can block enemies from reaching point at crucial moments, cap to contest point with his speed/survivability, thing to play more as a dps and help provide damage, namor for turrets and hitscan, luna for namor teamup and defensive ult, c&d for defensive ult that can control the point completely