Historically, tanks' worst enemies in the open have been anti-tank guns and aircraft (and other tanks of course). If in streetfighting or forest or something, then infantry.
Nowadays infantry-carried RPGs are pretty effective, however all the most modern tanks are exceptionally well armoured against all but the most modern RPGs. So it's a tie, more or less (except that RPGs are much much less expensive
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Infantry isn't that strong against tanks in the open, but in built up or forested areas it has the upper hand. Too bad that can't be well modelled in Generals (and in nearly every other similar game I've played, except say Blitzkrieg or the serious tactical games like Close Combat or Steel Panthers.