the witcher 3
i go in expecting something regular-good like shadow of mordor and end up playing a top 3 game of all time

It was a while ago, but Xenogears. I hated Mechs. Anything that had mechs in it, I hated it. Now Xenogears is definitely in my top 10 RPGS ever. And after playing Gears, I gave Xenoblade Chronicles a try and now Monolith has me by the balls.

Agree with you on Mario+Rabbids. Other games that comes to mind right now is Dragon's Dogma and Bloodborne

A.O.T.2 (Attack On Titan 2).
I got for the switch the day it came out. I thought I might like this. I like the first season of the Anime and the developer but man I loved that game. It's one of favourite games on switch. It's one of my favourite games in a long time and I know it's got a few flaws but that only makes me want to see what they do with the next game!

World of Warcraft was a game that I talked trash about during the beta. I didn't want to play it, I thought it looked cartoonish and shallow. My friend kept bugging me, though, and telling me how much fun he was having with the beta. He spent so much time talking about it that I had to play it to at least say I tried it. And then ... I hated it. Leveling up to in the starting area was boring and all the "lore" just went over my head. My friend pushed me into trying the Battlegrounds and then I REALLY FREAKING HATED THE GAME. It was a massacre. The other team killed me as soon as I spawned, over and over. I swore I would never go back. When I told my friend, he said I'd had the bad luck to run into a "twink" guild. So, I kept leveling. Then, out in the open world, a Horde Rogue jumped me--but I won! I beat him and tasted blood for the first time. That bit of excitement enveloped me to the point that I ignored video games that weren't WoW for the next few years.

Fallout 3 was a game that I didn't really want to play. At the time, I was still part of the "shooters are lame" elitists, especially those who talked about how Japanese games were so much better. More importantly, first-person games had always given me a headache. I came across the GOTY edition on Amazon one day, though, for super cheap, and I bought it on a whim. I absolutely fell in love with it. It had the kind of vibe that I like a lot plus all that freedom. When I discovered mods, it was like an incredibly sweet cherry on top.

This is always a nice surprise, and far better the the opposite :P

The first Killzone comes to mind. I actually played it just to get ready for 2, which was critically acclaimed. I ended up loving the poorly rated one.

Borderlands comes to mind. I haven't really cared much for most FPS games in general in a long time, and Borderlands was no exception. I got the first two games for christmas some years back, and thought I might as well try them out then. I ended up genuinely enjoying both titles, but to be fair a lot of that is because I played them in co-op. If I had been playing alone I'm not sure if I would have ever even finished the games, but playing with someone else on the same couch made the games so much more fun than they would have otherwise been.

This is pretty interesting, I saw the trailer and thought the graphics looked pretty lame, but I thought the in-town part with Historia looked really nice. Is there a relationship or community system in the game? What kind of flaws does it have? I enjoy the manga/anime and so I can probably forgive some flaws if the fanservice is nice.

Also to answer the thread topic, I usually know the games I want nowadays, so I haven't felt this in a while. I do have plenty of games where I think I'll enjoy but end up loving, or think I'll love and end up not caring much for, but usually I have a good idea of what I'll like.

Thinking back though, I'd say Fire Emblem on GBA and Tales of Symphonia are good examples of games that I got on a whim and ended up as some of my favorites ever, both currently in my top 10. Even further back though, Legend of Zelda: Ocarina of Time is basically the game that turned me into a gaming hobbyist, so that one kind of wins by default.

Overwatch. Since it was a first person shooter, I thought I wouldn't get into it. The beta went by and I had no intention of playing it, then I saw my brother and some friends were playing it a lot. I gave in and bought the game, 2 and a half years and 600 hours of playtime later, I'm still hooked.