I'm guessing a bit here but in order of confidence: 4th armored, 15th Scotts, and Canada for the allies.
For the axis, Falschies, 12th SS, 21st
Reasons: 4th armor is simply amazing overall with the sole exception of lacking a heavy tank. While I find a methodical advance of Panthers, tigers, and jadgepanzers to be invincible against the 13 AP of the 4th armor, usually I can just trade space for time since I'll have a big lead in points. It has good amounts of infantry with the choice between halftracks and trucks. It has 105 Shermans in phase A and a vet 2 leader Sherman in phase A. The B26. I've seen a trio strafe a Jadgepanther to death and 2 of them can fend off a fighter. 600kph recon plane. Arty is a bit light but adequate with the Priest and M12 and MHC.
15th Scotts are slower but the AVRE and again, pretty good balance between tanks and stuff. I haven't played them as much. Canada less so but same reason. The Ram command tank in phase A is pretty good if light on machine guns.
Falschies have large amounts of veteran infantry with double machine guns, HS 129, panzer L6, and that fucking phase A offmap. Their Arty gun is good too. I wouldn't want to face an armored division in the open, with the 8 AP AT and all. But that AT is vet/double vet and with some air control and the 129, you could reasonably contest an armored division.
12th SS and 21st panzer are both simply well rounded tank divisions with weakish but competitive early games, large amounts of panzergrenadiers, medium armor in phase B, and some heavies in C. The 12th has the firefly, which is better for team games (200 price for an armored mobile AT piece in phase A) but can lock down, say, 4th armored in the open. 21st has to play with those halftrack pak things but they're cheaper and all. But again, it boils down to large amounts of panzergrenadiers.