Rattata/Raticate is (I think) the original FEAR Pokemon.
But these are some other ones
Doduo
Dodrio
Hitmontop
Treecko
Grovyle
Sceptile
Taillow
Swellow
Starly
Staravia
Staraptor
Archen
Archeops
There really isn't any "Best" FEAR Pokemon, it's just whatever works best for your team.
It depends on the generation which Pokemon is easiest to find. Gen V it's almost impossible to get the Pokemon I mentioned above before reaching the elite four, unless you can trade.
Source:
http://pokemondb.net/pokebase/111917/how-many-fear-pokemon-are-there