Of the ones I own, I'd say:
Ridge Racer: Content light and 30FPS, but pretty damned enjoyable if you're into time trials.
WipEout: Shit hot, but you own it.
WRC3: Shit.
Asphalt: Worth £6.29, brainless arcade racer. It's very 'Gameloft', but it does the job.
Moto GP13: Cracking port but difficult if you're not into bikes.
MUD: Same as above, but on dirt!
NFS Most Wanted: Fucking great. Love it, love it, love it. Can't recommend it if you'd rather play on console though.
Sonic Transformed: Cracking little port, cracking little racer.
To be honest, you already own the best racer on Vita. I'd say Sonic was a good choice if you get on with kart games. NFS if you're not bothered by the graphical downgrades. Asphalt if you don't mind playing a cheapy Gameloft racer. None of them leap out to me as 'must recommend' games though, sadly.