Depends on what you want to do. If ranged dps, probably pyrotech powetech (can tank too if you want to) or mercenary (middle skill tree aint bad with merc either). If you want to melee dps, then marauder (first skill tree is most popular right now). If you want to heal, operative (stealth and burst dps ain't bad either). Sniper ain't bad either if you want to be more stationary and dps. Though with 1.4 assasin and sorceror will be able to sprint away more.
I will start that for ranged you would want to go sniper/gunslinger, PT/vanguards are getting brought into more of a 10m range and merc is eh (I have seen numbers but not that often). Mara is probably the best melee dps but assassins can put out good numbers but they have to work for it and are more squishy unless you play in their tank stance and know what you are doing. Op heals are top right now but I play with sorcs and a merc healer that are just awesome at healing.
TBH though in hindsight I have seen every single class perform great you just need to know what you are doing. Just pick a class YOU want to play not one that people see as being "OP".