Most new games I pick up on a whim?

I guess most notable is the most obvious:

1. World of Warcraft, got the game with dial up internet because the boxart looked cool, had very little concept of online gaming at the time. Life changed for years.

2. League of Legends, started playing it because my friend started playing it, best $0 I've ever spent on a game.

Definitely Fire Emblem Awakening! I had never played a FE game up until this point and this game single handedly made me play my 3ds far more than any system that year. It's one of the few games that I beat so many times that I lost count. Nintendo needs to allow more of its developers to do what Intelligent Systems did with FE. Make the game as if it's the very last one that's going to be made! Couldn't have been a better starting point for why that game turned out so great... 

The original Forza Horizon and 2 had me hooked. I'm not even a big fan of racing games.

Rogue Legacy also had its hooks in me in spite of giving me a bad first impression.

I'm with you on Dishonored.

I remember watching a video when it was released.  The guy used a rat swarm to kill someone.  I thought the graphics were dark and dreary, that the people looked wierd, and that the combat was bad.  A long time later it came out on PS+ and I decided to try it.  Once you start putting powers together it really becomes something else.  I had a blast slinking around, killing people from the shadows.  It's a game where videos do it little justice.  Can't wait for Dishonored 2.

Far Cry: Blood Dragon is a game I thought it would be a simple little spin off that couldn't compare to Far Cry 3.  Turns out that I actually had a lot more fun with it.  It's not nearly as pretentious, the scale of the island is more enjoyable, and most importantly, the game-play is superior.  Specifically, taking bases is much improved over Far Cry 3, which happens to be one of my favorite elements of Far Cry games.

Might & Magic: Clash of Heroes was an even bigger surprise.  I don't even remember why I bought it.  Probably really cheap during a sale or maybe it was PS+, as well.  Regardless, I absolutely love that game.  I've probably beaten it four or five times.  I don't know why, exactly.  All I can say is that it's just a perfect kind of game for me to relax with.

The last one to happen was Red Steel 2.I got it as a present and I had no idea whatsoever what this game was about.I actually had never heard about it.But sinceI got it for free I though:"Heh, why not?".When I tried, you have no idea how glad I was.It is really great to go completely blind, and I mean completely, and discover that a game is excellent.

Oldest one would be Final Fantasy 7.

I borrowed my kid brothers' Ps1 just to see what games there were to play, never had a console since the Videopac 2000. It came with Wipeout and FF7 and 8. I was done with Wipeout within an hour and then started up FF7, not knowing what to expect...
100+ hours later i woke up, so to say.

Then, World of Warcraft.

Introduced to me and a friend by a friend. Never really had a notion of this online crap. Tried levelling a bit and ended up ruining the better part of my life ;p.

Comes in Far Cry 3.

Having played Wolf3d in the nineties, followed up by a bit of Doom, to me Fps's were generic, boring, same shit different day kind of games. I guess they still are. Far Cry 3 however I platinumed and very much enjoyed its' gameplay. I will probably enjoy the next Fps as much in a year or ten.

Phantasy Star Online 2. Thought it was dumb and told my brother to take it back where he got it. He said ok when he gets back from being outside. Got it from his friend. Well for some reason I put the game back in and couldn't stop playing it.

Akiba's Trip. Who knew beating up and stripping the clothes off of people could be so much fun...?

Also, it's a good way for me to relieve stress on cops.. xD

CoD Black Ops 2. Not really an addiction but it was my first CoD and I played a lot more of it then I ever expected. Sadly also my last CoD, Ghosts and AW were garbage.
Pokemon Black. This one wasnt unexpected after putting 1000 hours into Diamond but I averaged between 11-12 hours a day on Black in its first week. Nothing will come close to that again... thanks to having a job mostly

My real answer though goes to Rocket League. Sure it looked like a cool fun idea but never did I expect it to become my most played game of the generation by quite a damn lot. Over 300 hours on Steam and roughly 75-100 on PS4 (why doesnt PS4 track time played )

The atelier series