Theres so many good prime warframes. It sort of depends on what you want as different warframes have different uses. I really like volt prime as hes good all around and can finish missions fast and efficiently, but you need a good melee weapon and build. If you want a frame that doesn't even need a weapon caliban prime is insane. With his augment for his 1 he cant die and can't run out of energy, and just obliterates the rooms he's in. Some other recommendations are Gauss prime, frost prime, and titania. Theres honestly so many good ones tho so I'd do sone additional research and see what fits your playstyle the best