Tales of Vesperia, Lost Odyssey, and Persona 3 definitely for me. I was skeptical and even downplayed them for various reasons. I remembering not being very impressed with the Vesperia. Once I really dived in to these games, each turned out to be one of all time favorite games.

Mario Sunshine, Mario Galaxy, Mario 3D World and Skyward Sword.

I guess Mario never looks that appealing to me but I often enjoy them in the end.

Command and Conquer: Red Alert back 20 years ago.
Originally I though "meh" Just another Command and Conquer with a new skin... But holy crap I got addicted fast.

Demon's Souls

Infamous, thought it looked meh, but it has some of the best gameplay mechanics in any game, the traversal and combat are so smooth and it had a lot more content than I expected. The story also started out meh but became a lot more interesting as you played the game. This game didn't want me to leave my PS3 for awhile lol. And that's while its the weakest entry in the series.

Killzone 2, thought it looked too generic, but it had a great atmosphere, graphics and feel to it that I didn't expect from it. The level design and AI were amazing. The game did became a bit tedious after a while due to the lack of colour palette but it was still a very good game.

Killzone 3, when I became bored of Killzone 2 because of its brown colour I decided not to play the next game for sometime, but it went on sale so I picked it up and it amazed me again with its graphics, gameplay and much more varied level designs and weapons. The jetpack was really fun to use and the forest and snow levels were really amazing and the game felt a lot more exciting than the more methodical and tactile combat of Killzone 2. Both are different enough to be great in their own ways even though they belong to the same series.

GTA IV, the world was a lot more immersive than previous GTA games and the story was great. I had heard mediocre things about the game on the internet before I played it but I knew I had to play it as a GTA fan. Definitely a very underrated game by gamers and overrated by critics, but still a very good game that has its own strengths and weaknesses compared to other games in the series.

GTA EFLC, it had all the strengths of GTA IV but with better missions, better bike handling physics and with the connection of its two protagonists with Niko Bellic it is better than GTA IV and makes GTA IV feel more complete, with the interconnected stories of three protagonists with different backgrounds and crimes in Liberty City.

NFS Hot Pursuit, a very thrilling racing game that was a lot better and addictive than I had expected. Ther gameplay had depth, physics were great and AI was challenging but not overwhelming and the soundtrack and setting were amazing.