Metroid over Castlevania.
First Metroid game was Metroid 2: Return of Samus for the Gameboy, at the time a decent and good entry into the series for my 5-year-old self.
My next Metroid game was Metroid Fusion, followed by Metroid Prime a month later. These are my two of my favorite titles in the series. When Metroid Prime 2 released, I received it less favorably than Metroid Prime but have since come to appreciate it more than the first one. It's definitely a game that, once the initial impression wears off, finds itself to be a better product than its predecessor.
After the Prime games and Fusion, I played Metroid: Zero Mission and was blown away (had never played the original), albeit I found it rather short. Nevertheless, it became my fourth favorite game in the series.
In 2012 I played Super Metroid for the first time and while I found it fun, I did not enjoy it more than every Metroid game I had played up until then sans Metroid 2: Return of Samus. The float-like movement didn't resonate well with me and because I had already experienced Zebes in Zero Mission, I was not interested in visiting the same planet twice (though the differences are noted).
I'd love to play Metroid Prime 3 but I was admittedly turned off during its reveal because of the Wiimote usage. Oh, and I also played the Another Metroid 2 Remake a few months ago and while that was fun, it showed how glaringly flawed Metroid 2 was. I now retroactively appreciate that game much less than I did as a child.
Compare my experience with Metroid to that of the Castlevania series where I've played a handful of GBA games and only ever completed Symphony of the Night (which I find to be incredibly middle-of-the-road), and it's no surprise why I prefer the former.