Are you talking straight line speed? If so then these are the fastest cars in GTA 5.
1st: Ocelot Pariah (136 mph)
2nd: Pfister 811 (133 mph)
3rd: Principe Deveste Eight (132 mph)
4th: Bravado Banshee 900R (131 mph)
5th: Invetero Coquette D10 (130mph)
6th: Överflöd Entity XXR (127 mph)
If your looking at lap times then the list is very different. They are always adding DLC packs though so this list has changed over time and may well change again.
My fave all rounder is the Dewbauchee Vagner and can win most supercar races using that one cos it corners like it's on rails.