I like 3 equally for 3 different reasons...
1. Drac's Curse, for fucking rocking hard and mastering the action genre on limited hardware, pushing it's graphical, musical, and gameplay boundaries to the max.
2. CV4, for take everything from part 3 and taking it to the next level. Haunting music, dark atmosphere, and lots of little animated nuances made this a masterpiece with plenty of replay value.
3. SotN, because of the same reason as CV4... taking it to the next level, while still being loyal to the mechanics that started the series.
So that may be a cop out because I can't narrow it down, but so what? SotN is probably my overall favorite, but I prefer CV4 because it rocks and I can play it through in one sitting. You have to get involved with SotN, make a several hour (or day) commitment to the journey... And sometimes I just wanna kick ass through Transylvania.