Personally, I'm kind of locked in the third gen. It's by faaar my favorite gen for many reasons, and I can't stop playing them. I bought back all the cartridges, two GBA SP and a link cable. I think I might be a 3rd gen "genwunner" because of my true love for this gen, but I still like pretty much all the generations. Well, except for the first one, since Fire Red & Leaf Green made it pretty useless... Though I feel like I'm going back in time since I've bought Pokémon Crystal recently, and I enjoy it a lot. xD
I don't think my mind is that closed, since I've enjoyed every Pokémon game, each one has its pros and cons, along with its own nostalgia feeling: 1st and 2nd gen because I was a kid, 3rd gen because I loved Hoenn as soon as I began to play Ruby, 4th gen because of the beginning of the battles and exchanges through the world (that, and the fact that I got my Internet connection for the first time in 2007, the same year I got Diamond)... I even like the Pokémon Colosseum and Mystery Dungeon spinoffs. But it's obvious being in love with something (or someone?) makes our point of view lose its objectivity. Many people hate the 3rd gen because 2muchH2O, and they're right, yet it's one of the reasons why I love it.
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I'm also a fan of another popular game series: Halo. Like Pokémon, there are many episodes in the main series, 5 for now, and there is gonna be a 6th one soon. I've played these games since I'm little too (oooouu, he's not respecting the PEGI 16 label!! what a thug! you only yolo once), and what made me love this game is the story, along with the very simple gameplay, quick to grasp. As long as Bungie was in charge, this was okay and the three first games were awesome, my favorite one being Halo 2 (props to 343 Industries for the beautiful remake). Bungie also made the spinoff Halo: Reach, though I didn't enjoyed it as much, not only because of the absence of John-117, but also because I feel they tried things which didn't turn out useful, making the gameplay a bit more complex for nothing.
When the series began to become controversial, is when 343 Industries took control. A lot of players rejected them as soon as Halo 4 was out. Halo 4 is a game with awesome graphics and soundtracks, but that hasn't Bungie's footprint anymore, especially in the OST, and that's what made the players frustrated. While I do agree Halo 4 doesn't feel like a Halo game, that's not all. The story, what a lot of fans think it's the game's forte, started to become absurd, with for example the return of the Covenants as foes, which I can't help but think it was forced. As a whole, the game is still awesome, but it doesn't have the Halo feeling anymore, and it reduces a lot the fun capital of the game. Maybe the franchise is only evolving, like a Pokémon who reached the level 36? I don't know. Humans find hard and tiring to change, that's probably why they can't bear it. I'll have to buy Halo 5 to make my own idea on how the story is moving.
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Just to say that sticking to an objective opinion about a game is hard if we're a fan. Like I know how much a game is awesome, though I can't find many opportunities to personally enjoy it because I have my own reasons. But as long as we understand and know there are many different opinions, and we accept them, that's okay. Just don't expect to debate peacefully on YouTube and such, because you won't find many open-minded players around there.