The essentials are:
Castlevania's 1-3 (Simons Quest is kinda

since you HAVE to use a guide to play that game on order to finish it cause of poor translations)
Super Castlevania 4 and Rondo of Blood are must play's if you want that linear style of gameplay, you can get both on the Virtual Console (SCV4 is on there, dunno about Rondo)
Symphony of the Night is the gateway Metroidvania game, couldn't recommend this game enough if you like the Metroid style games. Not really challenging like the previous Castlevania's but the amount of content in the game makes up for it's lack of challenge.
The GBA games Circle of the Moon and Aria of Sorrow are very solid games that continue with the SOTN style gameplay and expands on it. Oh and Harmony of Dissonance too but im not really a fan of that game, thought it was

.
The DS games are some of the best game's out on the platform, Dawn of Sorrow and Order of Ecclesia especially.
Your mileage may very:
Castlevania Legacy of Darkness for the 64 fixes some of the issues the vanilla version had, still not really a great game but I'd still give it a look if you have a N64
Lament of Innocence for the PS2 is basically diet Devil May Cry, it's a good action game but it level design get's repetitive.
Curse of Darkness, same as Lament of Innocence but with worse level design. At least the music is good.
Lords of Shadow, the ever controversial one. If you don't mind the God of War type gameplay with QTE's almost everywhere, you'll find some enjoyment out of the game and it's also long as fukk for an action game.
Lords of Shadow 2, this one is hard to recommend. I'll just say that it improves on the combat, bosses are fun and it's one of the few games that let you turn off QTE's if you want, plus it's like a 3D Metroidvania instead of a linear game like LOS1 was. It's just that this game has a lot of bullshyt moment's that'll kill the game for you. Rent it with extreme caution.

tier:
Castlevania Judgement, that's it. The less I talk about it the better.