These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Anti-Climax Boss: Some consider Dracula's One-Winged Angel from the first game to be this, since all it does is jump around and breathe fire at you (in other Castlevania games, it would at least fire projectiles while jumping!)
Game Breaker: Maria either counts as this or as an Easy mode. She's faster than Richter, she has a double jump, her jumps are much easier to control, she can throw two doves at the same time, which can hit the enemy up to twice, she can slide, she is slightly smaller than Richter, which makes a notable difference when you want to dodge certain attacks, and her sub-weapons, although different, can be just as deadly as (If not deadlier than) Richter's when used well. Her ending can definitely count as Easy-Mode Mockery though, as it's far less serious and far more cutesy than Richter's ending.
Magnum Opus: This is often a candidate for defining game of the series, and Toru Hagihara as well.