Exotic, exotic modern, classic, unobtainable, in that order. You'd probably get different answers tomorrow.
1. Britten (I've seen one for sale, so get to include it here.)
2. F4 CC
3. Velocette KTT or Norton Manx
4. A Joey Dunlop, IOM-winning factory racer. The bike he rode in "Man to Man" for the filmed lap of the island would be a real bonus.
2008 MV Agusta F4 312 R
2002 Ducati ST4S
1998 Ducati 748
1994 Ducati 900SS
1980 Yamaha TZ250G