I've been using mostly Motiv ever since I received a GT1 at the BTM Super School. In my opinion, they have always made very good cores, but the coverstock on the original balls (MOTIVator-Z, T, X, R) weren't that great. Although I did and still do love the GT1 and SR2.
I found the Vicious and Formula covers to be a nice improvement but once they started producing the Atomix and Whiplash covers, they really became a player.
The new generation of coverstock's have been outstanding.