How many cores would you like? 4? 6? 8?
Well, I should clarify first. I'm a realist and I understand the iMac is an all in one and there is nothing currently available to run near the TDP required for the iMacs current design. An i7-6850 (6 core) has a TDP of 140w, current 6700K is around 90w I believe.
Also assuming there was some ground breaking redesign and the iMac could keep an Intel 6950X (10 Core) cool I wouldnt be willing to pay an additional 1500-2000 dollar premium.
But to answer your question assuming cost and cooling weren't factors as many as possible while still maintaining a 3.5ghz or greater frequency with turbo boost at or around 4ghz. AKA Pretty much Intels Extreme Edition line.
Cooling is equally as important to me just for noise. I currently have an i5 which even with all 4 core at 100% doesn't lift the fan RPM above 1200.
The i7 is much faster especially with HT but to me its not worth it if my iMac sounds like a Hoover.