For me it's probably Stronghold.

Mainly because this was back when EBgames used to take returns for money (pc games at least), however just when they started making the whole "if it's not open" rule.

So I had bought a game that didn't run on my pc, it was open and they wouldn't take it bak for a refund. So I had to pick an exchange of another game. I ended up picking Stronghold because it was a new game and I planned to simply take it to another EBgames and get a refund.

So After I finally got done at the first one where I originally bought my game I went about 3 miles to the next EBgames to get a refund finally, or at least I thought. I walk in the door, tell the guy I got the game as a gift and wanted a refund. He looked at the receipt and then at the box, low and behold the bastard at teh other EBgames CUT the tape on it, I never even really looked at the box let alone tried to open it.

Needless to say, I was pissed. I bought a game and paid my money for it, it didn't worrk on my pc, honestly and here I was, stuck with a game I never intended to buy or wanted at the time.

So I finally went home and then a few days later I decided that since I was stuck with it I might as well try it. Man, was I glad that it turned out like it did, I loved the game.

It quickly become one of my favorite RTS games and still remains the best "castle defense" RTS games out there hands down.

Just a shame that they couldn't make a new one a la the Total War series but with thieir awesome castles and sieges style, it would be so awesome.

It just seems like they gave up on the series now after the last few games, a pity really. They went from the more middle ages historical stuff to a fantasy thing, didn't really work for me.

Mass Effect 2.

I always found the original game to be mediocre at best, with a very generic plot and characters we'd all seen a thousand times in Bioware's earlier games (the first squadmate you get even has the same VA as the one in KotOR!). The combat was dull and the choices between paragon and renegade paths were really just a choice about saying either "would you please do what i say" or "do what i say or i blow your freaking head off".

The sequel left me very pleasantly surprised. The combat was now more action and less sitting behind a wall waiting for your superpowers recharge, without being any less tactical. The squadmates were actually interesting personalities that you wanted to keep alive through the mission. The story was still mostly the same, but it had been hidden in the background and the focus was on the squadmates' much more thought-provoking backstories. Mordin's loyalty mission was spectacularly good.

So, a very average RPG-shooter was turned into a really good candinate for the game of the year 2010 (the only proper challenger for it being Heavy Rain, btw).