It seems to be personal opinion really... but the major ones seem to be qik, tango, fring & yahoo.
i use qik simply because it came with my EVO and after the recent update (v0.4.60.2) it works well with my friends and family that also have EVOs. I've heard that tango works well and is cross platform meaning it can work with iphone's facetime which sounds pretty awesome. i haven't used it though....