Every time I go to SD I see more and more places that look promising, but the ones I can vouch for are La Cuchara de Madera (Piantini) and La Patisserie (Bella Vista).
A good quesillo would probably be found at the more traditional, Dominican-style reposterias like Bondelic (Urb. Fern?ndez), Del Jardin (Evaristo Morales) or the one in Seralles off Jose Amado Soler, De La Casa.