Heavy Cavalry is almost a must, it just offers so much of value when it comes to riflemen defenses, smoke and of course the Pershing is really good with support from other units.
Rifleman and armor are nice for flamers, but I'm more partial to rifleman. Some maps are just garrison hot spots so it's good to add the extra hot with non-doctrinal flamers.
Mechanized is really nice for the WC-51 as it can bully both the Wehrmacht and OKW pretty well, and it's dirt cheap. Not to mention you can (risky) capture points with it early by hopping the crew out and getting some extra cap pressure.