Full Metal Panic
There's three seasons to this anime and all three contribute to the greatness of this series. The second season has no real plot and is mostly a series of gag stories but it's funny and gives more insight into the characters.
The first reason why you should watch this anime is simply for the beautiful OP/ED songs. Mikuni Shimokawa sings a song in each of the three series if I remember correctly. And her voice and songs are magical. She's just so good.
The anime of course is also entertaining. I don't remember the genre but it feels Seinen. The main setting is a high school. This soldier is a high school aged person named Sagara Sousuke. He was trained as a child soldier and has exceptional combat abilities and no common sense.
His mission at the high school is to protect a specific student with blue hair named Kaname Chidori. There's quite a bit of mecha thrown in because military units battle in armored suits.
I think there's certain character types that I really enjoy.. and that's the OP beyond imagination type of character. Like Kira from Gundam Seed. It's so retarded how Kira got beat by Shin, but I digress. Sagara is pretty much the uber tier in terms of soldier level and you get to see him do some crazy shit in person and in a suit.
I'm tempted to download and rewatch this series but I've seen it probably over 5x as a kid. You should definitely watch this series if you haven't already. The music alone is worth it.
tl;dr - amazing Mikuni Shimokawa music. OP main character with good characters, plot and comedy. Go watch now