Depends on application and price range. If venting directly outside, 6" or 8" Vortex or can fan. If going from growtent to the room the growtent is in, an inline bathroom fan works.

Can Fans
Vortex Fans
Bathroom Exhaust Fan

Or, there's those attic vent fans (12-48") but that's likely overkill.
Rusty Trichome Reviewed by Rusty Trichome on . best fan for exhaust purchasing gear for my first grow-room- need advice on exhaust fan- room will be 640sq feet-6" exhaust duct- nee d good fan , hopefully quite and affordable- have sen a lot of different fans, read all sticky's, still would like individual personal experiences and preferances Rating: 5