Here's a list of what I think is the order of best to worst PvE class (I like making lists :]):
Medic - Theoretically endless solo capability. Slowest, but surest class.
Whipper - Able to take on multiple enemies at once, the best. Racks up the most kills in the shortest time (especially at later levels).
Punisher - Rips through enemies - the quickest at high health, but a single-target only class (aside from it's mech).
Defender - Second slowest class, able to outlast all but Medic in PvE. Relies on living long enough to survive elite monsters (the ones with significantly more HP than the ones around them).
Shadow Walker - Rests often, but easily takes out enemies while at low health. More reliable kills than Punisher. A mix between Whipper and Punisher, but closer to Punisher.
Sentinel - A little slower than Punisher, and needs to rest a bit longer than Shadow Walker.
I would say Medic, Whipper and Defender are the best at later levels, where most of the monsters are clustered together, and a lot of them are aggressive. Medic is the worst at early levels, but it shouldn't matter once you get strong equipment. Same for other classes.