gta  

I say so far for the PSP, Vice City Stories (in my opinion) is the best.
Mostly because it's set in Vice City. heheh

When I had a PSP, I had both GTA: Liberty City Stories and Vice City Stories. I must say, I absolutely adored them both, and loved the fact that you could play one of the most open-world games on-the-go. If you have the money, I would honestly get both simply for the two different storylines, environments etc. If not, go for VCS, as it offers everything in LCS plus a whole lot more