Nothing is too unreasonable when it comes to dream cars, that's why they're 'dream' cars, therefore my top 5 would be (not necessarily in this order)...
1) Ferrari F430 Coupe/Spider
2) Lamborghini Murcielago Roadster
3) Porsche Carrera GT
4) Pagani Zonda F/Roadster
5) Ferrari 360 Spider
Bugatti Veyron, Ferrari Enzo/F40/F50/355, Lamborghini Diablo 6.0/1999 Diablo VT Roadster, Aston Martin Vanquish/DB7 Vantage, Mercedes SL55/SLR, McLaren F1 and any Porsche 911 would be awesome, as far as Mustang goes a 1967 black fastback would be amazing, and of course Elenore or perhaps a black 2006 GT coupe