These builds offer a chance to win:
Early 3 warrior + support - given that the warrior bonus is 10 armor at 3 warriors, and only 15 armor at 6 warriors, it is incredibly flexible
Early 2 knights + support - knights supporting each other give a very large benefit, only 2 being needed makes this another flexible build
Early 2 druids + support - it is impossible for most builds to beat the two druid synergy early game
Any combination of the above is going to lead to a win streak, which will boost your economy and result in more snowballing.
Any combination that doesn't involve the above is going to get slaughtered. Assassins are laughably weak, anyone that places their units in the back 2 rows will see assassins dive into their front line and get slaughtered. That isn't even mentioning how weak their units are, Sandking gets my vote for weakest unit ingame, it has terrible DPS, incredibly low HP, and noone cares about 10 armor on a deadbeat like that.
Trolls are obviously great support, but so heavily contested I tend to avoid them unless I can start out with a bat rider which allows the knight path. I will buy batrider over any other unit on turn 1.
Mages get hard countered by scaled units, which can be incorporated into any of the winning combinations easily. Warlock healing builds are cute, but just don't cut it. Trust me I tried. Hard.
Stuff like savage requires 4+ units to get significant benefits, which is simply too much unless it is incorporated in one of the winning builds. Elusive is nice at the 9 unit mark, but at that point you are already fighting for the top 3-4 positions, and a better build would have already secured the first place.
Getting lucky with items or RNG can change the above, but it is rare.
Game needs some updates to prevent it from becoming stale.
Knights, Troll, Warriors and Mages are all top tier and viable with Warriors having a huge variety of end game builds to incorporate 3-6 of the other builds. I'd say you could win with anything as well if the RNG gods favor you and you get 5 3* on the board.
Mages are absolute ass tier once you meet people that understand how scaled units affect them.