Fedor is still the best fighter in the world right now in my opinion, just because he lost 1 fight in 10 years does not make him 'out of his time' at all.
As for Fedor in the UFC, that would be very interesting because some of these upcoming guys could really give him a fight like Dos Santos, Schuab, Velasquez.. it would be very interesting though, only time will tell.
As for Overeem, think Alistair would kick ass in the UFC.