which clutch has smoothest engagement?
If you don't want to swing the cash for a Centerforce, get a diaphragm style "stock replacement". (A diaphragm style wasn't stock but you can get them in a stock configuration).
Stock replacement clutches normally have more "Marcel" than race or high performance clutches. The amount of "Marcel" is what gives the smooth engagement.
Do not get a Borg & Beck (factory, 3 finger style). Borg & Beck style clutches give a heavier peddle and in some brands, a higher failure rate.
With that said…. Get a Centerforce if you can.