Tia Pol; Boqueria, Casa Mono and Baj Jamon are the usual contenders. They're also all but impossible to get into on the weekend, Casa Mono takes reservations and is more of a sit down 'normal' restaurant than the others. Boqueria might, or might only for larger groups. Tia Pol reserves for six or more.
Less obvious responses include El Cid (one of the original tapas spots), Oliva, on Houston, La Paella (the old 9th street standby, there since the '70's) and one can always fall back on the garlic heavy old school Spanish haunts - Sevilla, etc. There's also the place in the spanish cultural center..
Pipa can be a good date spot - most women in their mid-late 20's, through the mid-late 30's are familiar with it, comfortable there, and feel good about themselves in the light. The food is decent, but doesn't really compete with any of the places above.