Do we ever see tanks in the Fallout games? I can't really recall.
The divergence between our timeline and Fallouts is after World War 2, so they must have had WW2 tanks, but I'm not sure if there are any that you see dotted around destroyed in the series, or mentioned in text.
Well there was a Sherman in Tactics. Others were implied to exist. 
I think the XM-1 would have been a good retro scifi tank

as its not too retro looking and not too scifi looking
MBT-70 comes in at second place

Power Armor: The Enclave had T-51Bs in reserve while they produced the newer Advanced Power Armor.