Delta definetely. Been to almost all of them for pricing...DL has the best pricing and reservations system out of all of the ones I have tried (CO, UA, AA, SQ, TWA, BA, US). Also, DL's site offers many many options including 360º virtual tour of BizE...stats on avg plane and fleet age...and easy to use seat assignment/aircraft config models. DL has won awards for best "websmart" compant or something along those lines for their website.
BA's site is good too...and UA's has some good points too.