Biggest personal achievement? A bit hard to say...
Obtaining the "Loremaster" title in WoW (before I quit the game) was one of the bigger challenges I set out for myself, and is one of the few remaining positive memories I have about that game; it was good fun tracking down all of those little quests, and sometimes the stories behind them were interesting. However, it's somewhat tainted by my overall memory of WoW being a game built to drive obsessive behaviour patterns.
For something else, one of my personal bests is the completion of the first major
Skyrim mod project I set out to do. It was a fairly large dungeon mod, "Sanctum of the Fallen" [http://skyrim.nexusmods.com/mods/19913] I ended up calling it; ended up being something of a minor hit, but I would say what I'm most proud of about it is the fact I got something of that scale done purely for my own interests. I'm half-planning the follow-up to it, which is intended to be significantly larger, but my current schedule isn't exactly accommodating as it was while I was working on it. And may as well, the quick trailer I made for the mod is in the spoiler below:
There's probably other thing I'd call myself "proud" of, as far as gaming goes, but they're fairly minor in comparison to the above... not to mention my non-gaming achievements as well, but that's a different ball-game entirely.