gunner is my favorite hero and a very powerful one with some skill needed to use rail gun to full affect
tank is for holding points and is less useful on tdm games
scout is fast and has a hit and run play style and is very good at clearing caps and gettings kills, tho its harder to stay alive
tech is the main healer of the game and when paired with shocker amazing on holding the shuttle bay cap point, but less useful in tdm and moon base as you need to set up to be most affective and always need a team
blazer is a very high damage with a short-medium-long range weapon kit but damage past lock on range with the lazer rifle is very low(he also gives a damage over time affect that stacks up to 20 times)
assassin is like scout with a better hit and run but even less survivability as you have no movement abilities
blaster is like a mix between gunner and tank and is very dangerous in tdm and is also very good at holding points
shocker is much like tech but his healing rifle does damage but only heals sheilds, his q spawns a bubble shield and when paired with tech you can hold the shuttle bay point (when set up) for a very very long time (his secondary does shield damage and also disables shield regen for a set amount of time also stacking to 20)
bomber is also much like scout and assassin as it has low health but has even more mobility but his play style is more of a hover around a point shooting in on it to clear out bad guys (very fast and deadly but easy to kill if you get to close) (his secondary also has an affect that increases crit chance that also stacks to 20)
commando plays alot like a close-long range as nothing really is good at a medium range, with more survivabilliy than gunner as he has a regen abillity and his guns fire faster while under the affect, his secondary rifle has a scope that can headshot
out of these pick the one that fits your play style and wait for it to come to free to play for a day and unlock all of its abillitys then play with it untill you know for sure you want it then buy it