The question is near impossible to answer. There are too many variables between different boss fights. Some have lots of adds, some have none. Some have lots of health, some are quite weak in terms of health. Some bosses don't allow DPS to get close, whilst others do. The list is a lot longer but I'm not bothered
The best DPS power seems to constantly change based on updates. Don't worry about what is the best DPS power, focus on having fun.
The best DPS power changes every month, the only powers to stay up top and are still there are Ice and Earth. Quantum was up there too but I heard they broke it, idk how but I switched before this happened. Fire and gadgets were also top at one point, but fire is easily interrupted, and gadgets keep saying something about EMP messing up. HL was great at one point too when the first AM dropped, now it's rare to find one trolling let alone DPS. Top DPS powers aren't consistent, it will always change, just play something you like and have fun with.