Montana has some great cards but their team ability is vastly overrated. Uppers and All Stars have hands down the best TAs in the game. Uppers has the best support in both ELO and regular making it the #1 team with All Stars coming a close second. Sakhoram falls in 3rd on the TA scale, however it's lack of support makes it a mediocre clan at best. Though it's getting there.
(Yes technically Ulu Watu is #1 in ELO but I'm talking on an overall scale here).
Pussycats is the best support team in the game, and if Nightmare ever gets a decent lineup it'll be the best counter team in the game.
For the normal mode, (doubles included) Sakrohm has the best 2 defensive cards in the game and a reasonable attacker, but everything else in that clan is not really worth playing.
on an overall scale i have to agree with stormwind about the most consistent clans : Uppers (jackie included) and All Stars (in my oppinion maybe the best bonus in the game) 2 playable lvl 5 cards and 3 playable lvl 4 cards
As for the support team, Pussycats are really the most versatile clan, high power at low levels and the posibility to use them efficiently even at lvl 1, although Nightmare has a balancing bonus, they rely too much on abilities, in my view the best counter team is really All Stars, with Striker, Alexei, lamar and Flo, and with their bonus they really exploit any weakness