Topic: What game has left you the most satisfied upon beating it?

First time I beat the original Contra without having to use a continue.

Also, 999 after spending hour after grueling hour trying to figure out that horrible Sudoku puzzle near the end of the game.

Based on first completions, Xenoblade and Dementium II have left me the most satisfied. Xenoblade is just epic and engrossing and Dementium II's completion was so engrossing I beat it in one session on October 28th, 2012. I'm not going to bother with honorable mentions, as there are so many, from Super Mario Land 2 to Code of Princess.

Based on having multiple completions, Metroid Prime Hunters is easily the most satisfying game I complete.

I've never heard Metroid Prime Hunters getting that much praise before, is it that good?

Metroid Prime Hunters was actually a pretty awesome game. The online was great unless their was lag, and action replay eventually ruined online matches. Metroid Prime Hunters was literally the portable Halo experience(IMO. Also minus voice chat unless FC were used), except every character had their own traits.
Headshots worked, and the game looked great on the DS Lite(at the time). I wouldn't say it's as good as it was, but I got it the day it came out, and I liked it quite a bit. I only played through the storymode once though.
EDIT: That said, an updated Metroid Prime Hunters on 3DS would actually be a better than a Wii U entry, IMO.

Super Mario Bros. 3, if only because it was the first game I had ever managed to beat by myself.

Several other games since then (Majora's Mask, 999 and Persona 4, to name just a few) have left me feeling immensely satisfied with how the story was wrapped up and with how much I enjoyed the game itself, but at the time I defeated Bowser in SMB3, I'd spent a couple of year nearly beating several games and always fell just short, so I felt a measure of "getting the monkey off my back" triumph at the time.

There's definitely something about that game that controls so right in comparison to the first mario.

Any Nintendo game where the credits music is a remix of all the songs throughout the game, it's just so epic. Also, Kirby Super Star Ultra, after beating it completely (all copy essences, treasures, and helper to hero)

Ninja Gaiden (NES): I loved the game as a kid but could never beat so having finally beat it all these years later at age 32 is so satisfying.

Portal 2 and the first two PMD.

Warioware Twisted (unlocking everything)

banjo kazooie and banjo tooie becaue there my fav games and they were since i was like 4

Final Fantasy IX, the last boss is quite hard if you are unprepared, but it's a very, very good game. (now if only SquareEnix will remake it... preferable to 3DS...)

And recently, Kid Icarus: Uprising, although I still need to be able to defeat the 9.0 difficulty...

Also me and my bro beating Portal 2 multiplayer extremely fast, platinum trophying the game, and all without looking up anything takes major skillz.

Mass Effect

Bioshock Infinite had a very odd effect on me when I beat it. I hated the ending, absolutely hated it. I went to bed and couldn't fall asleep because I couldn't stop thinking about it. When I woke up I looked up a summary of the ending, and I actually understood it. The ending is so confusing, but if you understand it... It's so well done, everything is just perfect. It is the best story, and ending in all of video games (so far). For the next few days after that I could NOT stop thinking about it, It made me kind of sad really. I was actually sort of depressed for a few days. If a story has that much of an effect on you, you know it's good. It is now mine, my brothers, my cousins, and my friend's favorite game ever.

The Legend of Zelda Ocarina of Time. It is a long game, (Still very awesome) along with some hard objectives.