I've played all of them except the last one.

SotN is the best. Beelzebub is a great boss. The game has great atmosphere. The music in SotN is also very well done. The ending music is extremely cheesy though.

Rondo of Blood is the best out of the linear Castelvania games. Super Casltevania IV is the first one of the series I played. Overall, one of my favorite game series.