I'm a non F owner, but have driven them all (apart from the LFA), I see the F range as follows -
IS-F - Possibly could be a classic - but not sure it stands out enough. At the time it hit the market, it really didn't get the recognition it deserved.
GS-F - Most difficult to predict for me. As good as the RC-F - no doubt, but with 4 doors I don't think it has the impact of the RCF.
RC-F - Seems to be getting lots of good press recently. Being a two door coupe, I think it is a strong contender
RC-F Carbon - as the RC-F, but this car seems to be the daddy and the most rare. My money is on the RC-F Carbon.