Admittedly I've only ever played two Castlevania games... Castlevania 64 on, well, the N64... which I enjoyed at the time, not sure if it's considered a good or bad game by the fans though, lol. The second is Lords of Shadow, only the first one though, played it on PS3 then bought it in a Steam sale too, it was actually a REALLY good game... never got round to the sequel, but heard it was pretty bad or something anyway.
Never played any of the older games, and whenever I ask about which game to play, the answer always seems to be Symphony of the Night, but then some people also tell me it's a bad game too, lol. Guess I'll have to play it one day and find out, it's on the Playstation Store so I'll play it on my Vita... if I get to it, lol.
I think the fans think those 2 games are pretty bad. 64 particularly I think is quite infamous for being total trash, whereas Lords of Shadow seems to divide people between those who think it's great and those who don't like it just because it's too different.
Personally I got into the franchise with the very first game (I just posted about it a couple pages ago!). It's really great, even more so when you think it's a 1986 NES game, but it really holds up. Usually any game of that era is scary to get into for the mere thought of a game over, but in Castlevania you don't lose everything with a game over so it's actually pretty encouraging. The game is hard but almost all of the time it feels like a fair challenge and not one that's just overpunishing or wasting your time too much or anything.
I'm playing Symphony of the Night now, it's pretty damn awesome, but I did find it a bit too daunting coming in "fresh". I think it works better when you're already familiar with the previous games, or at least one of them.