Super Castlevania IV is the best Castlevania game ever made, so it wins. The better whip was a fantastic idea, and it's a real tragedy that they never brought it back in full. SCIV also has outstanding graphics too (sure it has slowdown, but it shows the SNES's power. RoB looks great, but can't do SNES effects.), great music (some of the series' best!), and a lot of levels, all very well-designed, too. It's also got a hard mode you unlock after beating the game.
Rondo of Blood is an amazing game that I also really like, though. I don't like playing as Richter much, since he's back to the annoying NES-style whip controls, but Maria is great. The game is quite a bit shorter than SCIV for a single playthrough, but does have some replay value if you want to find the alternate stages, though it doesn't have a harder difficulty setting, unless you consider Richter as hard mode and Maria as easy mode, but that kind of "hard mode" is an annoying way to do it I think. The game is a really good game, but it's not quite as good as Super Castlevania IV. Both games are far, far better than Bloodlines, for sure, though. Or Dracula XX (SNES).