Anything that's alternate dimension related like the Dracula's Void in SotN, He'll in DoS but Chaos Realm in AoS wins the match for just because it's the reminiscent of the castle stages in one area that you've already been familiar with with the add on of some new rooms that makes you feel that you're also in an astral plane with the stats in the background. I like how the time is stopped there, making you so eager to see what comes next yet give you a genuine sense of fear that you're now fighting tougher enemies that are also prone to time as well as you are. I think they should add more places like that in a castlevania game.
Also, I would like to point out that I like CVIII, SCVIV,RoB, OoE and LoS because you don't always have to start the game from the Castle, you can also enjoy the journey to the Castle. I praise RoB because from level one, you can decide whether if you want to go start the game from the castle or take the alternate path to the bridge and you can change paths in any level. That was some genius level design. I loved that game so much. While this seems a bit irrelevant because the Castle in LoS was not finale because the boss there was not the last boss, I like the idea of gameplay after the main course. So after the Castle, you still have more of the game before its truly over. That was a good innovation in the series and I'm they should use that idea again the LoS saga is over.