jammin: by far the best buggy: it has superior geometry and design (voted readers choice award 3rd for nitro buggy)
hyper 7 is 2nd but its kinda old (try a hyper 8)
if your gonna race, stay away from the ultra comp: it has limited adjustabitity and isn't a high quality buggy
i know there are bashers out there that prolly adore the ultra, but for racing, to put it bluntly, it sucks

if you can afford it, get a kyosho inferno 777, XRAY XB8, or a team magic M1B, but they're like $650 for the rolling chassis
but remember, racing is 80% driver skill, and 20% car/other variables

hope i helped