There is no “best” land unit. There are land units with different uses.
-When you want to defend actively, you want mostly infantry while having some attack units (which one depends on the kind of front that you are defending) to stop your opponent from going forward with a big stack.
-When you are all-out defense you want only infantry.
-When you are attacking you want a mix of attacking units and infantry, yes an attack as a bigger win% with the right mix of inf/tanks then with just tanks (there was a very nice post earlier explaining this for people who’d doubt).
-When you are all-out attacking well yes you’d want only tanks the only right scenario for that is blitzing Russia early with Germany and it necessites some prerequisites to be a fine option (in my own opinion, I know a good player who doesn’t agree with me there, depends on the play style I guess)
So in a nutshell, the most needed unit is definitely the infantry, but I wouldn’t say it is the “best” unit because it seems too much of a specific case question (although if you’d tell me that you’ll give me 20 bucks just to shut up and answer the question, I’d go with infantry).