Well, for prop aircraft, the 109 has it hands down as most produced from what I've seen.
In jets, the F-4 is definitely up there as is the F-86 Sabre
I'm still waiting for something trully DIFFERENT to land on hobby shop shelves.
I don't do much WWII at all, but if there was one thing above others I'd like to see kitted from that time period, it would be a 1/48 or 1/32 Petlyakov PE-2.
When it comes to jets, I have for too many in mind to mention here.