I think TOP THRILL DRAGSTER and KINGDA KA,but these are launched [sillyme]...not launched I think Millenium Scream and all the hyper that are near 100 meters of height[;)]
The fastest possible is several tens of thousands kph, but it's hard to tell exactly because it flickers really fast at that speed. I don't know the names of tracks that do that, but I've seen a few.
If you want a fast realistic coaster then I suggest you go and downloads Hansixx's "Thalys" or "Project Strata-Force"
If you want a stupid coaster which goes a billion miles per hour then be my guest and download the power tweaking and make all the transport sections of the ride to say 1,000,000,000 kph and have fun. See you on the other side.[hello]