Makai Warriors ("Dark Warriors") are much better for PvE due to their AoE taunt and a skill that boosts their defensive stats. I've made quite a few builds for tanking as them. Here are the threads they're in:
Dark Warriors and Dende Priest are the best picks for PVE classes. The other classes really can't compare to any of the other classes in areas of dps. Dark Warriors are tanks and Dende Priest are of course Healers. Just know if you play one of them they take a bit of work to level and there not made for dealing damage.
And what about poco summoners and shadow knights, where are those best at?
They are damage dealing classes but fail at it. They cannot compare to humans and majins in damage really. Shadow Knight however is good at PVP not so sure about Poco. Your best beat for playing a Namekian for PVE is to go as a tank or healer otherwise you will be pretty useless unless you wanna just PVP. But also sense they are so underpowered that leaves a lot of room for them to get buffed hopefully.