This is a rather hard question to answer, but most likely it'd be to relive the glory days of Tibia back in 2002-2003, back when it was completely new to me.
The experience was unlike any other game I had played up untill then. It was fresh, it was mysterious and you could get killed by anybody, meaning your reputation and who you knew matter more than anything in the game.
Other than that I'd most likely pick Dark Souls. From the very first 5 hours of playing it, it sucked me in so hard. It was such a fresh experience, in the midst of all the extremely mediocre triple a games came this polished gem out of nowhere and just slammed me in my face and said "You're my bi**h now, hear that?"
I've clocked nearly 430 hours since and I don't regret a single minute of it.
A strong contender might actually be Thief Gold, holy hot damn that game back in 2001 on my old w98 as a kid was so intriguing. The dark atmosphere, the amazing medieval/steampunkish setting and the general tone of the game (besides the amazing gameplay) makes me think that if I would completely and utterly forget about this game and play it fresh, I'd enjoy it just as much as I did back then.