Yeaaaah, I'd have to say Morrowind for one reason: DREUGE
My buddies and I all use to play Morrowind on our own, sometimes on multiple systems in the same building/room, and then tell each other about any good treasures/quests/dungeons/ester eggs that we would find. (we still do the same thing now in Obllivion and Fallout, its always awesome when you get to show off some new Mother Lode >

)
But anyhow, I had never gone in the water too much in Morrowind up to the point this story happened, mostly just crossing rivers, or some minor flooded areas in dungeons, NEVER in the oceans. But one day I'm haullin up the wester coastline, I check my map about every 5 minutes or so to keep track of my position and bearings, and suddenly I see this bay, not huge but still a descent dive, almost entirely surrounded by mountains, and I'm set almost straight for it, so i decide to check it out. I get there, but to an opening in the mountains, and have a look down the hole; place turns out to be a ruin of some sort. Now I'm thinkin "YATZEE" cause this is obviously a loot den, and I just dive right in and start lookin for a door..... I think you can see where this is going. -_-
My first encounter with a dreuge was conducted thusly:
I spot a vague blur moving fairly quickly toward me through the murky water, "what the hell is tha... OMYSHIT, ITS CTHULHU, GET THE FUCK OUT, RUUUUN..... T_T
And then when I get back to shore I turn around and the fucker is just right there, STARING AT ME...
So I took a few minutes to get used to the thing, and then jumped back in and kicked the living shit out of it >:O..... ended up making a candle out of the basterd (dreuge wax has 'light' as one of its alchemy properties XD)