Just vieja, or vieja and paratheraps species? Either way I like vieja maculicauda. Paratheraps brehdori a close second. Then a toss up with synspilum, fenestratus, and bifasciatus.
I like nearly all of them (and own quite a few), so it is very hard to narrow down to 3 (love synspillums, regani, argentae and bredhori almost as much! LOL...)