Honestly not sure who I'd pick, although I could see all of them retiring within the next 3 seasons.
We already know about Nalbandian's intentions, although obviously he might not stick to them.
Ferrero and Hewitt are probably closest to retirement out of this group imo...don't see either of them doing anything particularly noteworthy in the biggest events now. Much more emphasis in today's game on having a big weapon to call upon, and neither of them has one. For me, the question is how much longer they will each stay motivated. Probably a little more motivation for Hewitt (Davis Cup), given the lack of depth in Australian tennis, while it wouldn't take much for Ferrero to fall behind the leading pack of Spanish players, and out of sight out of mind.
I don't necessarily believe that Federer will continue to play until the 2012 Olympics...the Slams are everything to him now as far as winning titles is concerned, and I think that once he finally starts to come unstuck in some of the earlier rounds, he will be seriously contemplating his future. Maybe this might start to happen next year, or perhaps the year after...only time will tell.
I don't see Roddick continuing on for too much longer either.
Sure, his serve will keep him competitive for as long as he wants to play (barring injuries), within reason, but I think it is fair to say that he is on his way back down again (could have a fight to finish top 8 next year), and I question how much longer he could stay interested in that case.
Anyway, to give a definitive answer...I'll take Ferrero over Hewitt.