The 3 best fruits tie in my opinion. Mera Mera, Goro Goro, and Hie Hie.
If Doflamingo's power is DF based, then that's really powerful against weak minds; however, it's not that useful against strong villains who slay their underlings anyway.
Moku moku is good but I'm not sure what powers Oda has in mind for smoke.
I guess DF power depends on the user, so suna suna is really cool too if the sand can crystalize to glass (so water can't weaken it)
DF's are awesome toys for the creative mind.