Borderlands 2 is among my favorite games of all time. I'm thinking easily within my top 20. And it's definitely in my top 10 FPS games of all time too.
It's an improvement on the first one in every single aspect. It's the kind of sequel that I like to call "true" sequel. Not only a game that 'follows' the events of the first, but a game that improves from the first in as many branches, nooks and crannies as possible. It's also the type of game that makes it absolutely clear as you play and pay attention to specific game elements that the development team was dedicated to their project, that they really cared about it and wanted to make something good, and fun. The story, characters and the chemistry between them, the general writing is great, the villain is very memorable; the soundtrack is fantastic and fitting for the game, the itemization is much better, the difficulty levels are better balanced, the enemy variety is great... everything is better, period. I could go on and on about BL2 but it's one game that the 2010s gave us that for me is forever in my hall of fame. I had a blast in single player, and it's even better in co-operation online. I think I finished it around 5 or 6 times by now with my cousin, and we always end up coming back to it.
It's also one of those games that I consider essentially perfect. There are flaws, but there's so many positives about it that the "flaws" become negligible overall. There's a level of polish in the game that both BL1 and Pre-Sequel lacked, especially in terms of level design and layout, along with character skills and other 'minor' (polish-related) things and details that I'm the sort of person to pay attention to: like enemy placement (on the ones that are scripted to spawn), enemy variety, visual diversity in levels, banter between enemies or between them and your character, fine touches of intelligent writing especially in the dark humor from the villain, etc. I easily categorize BL2 in the type of FPS games that 'marked' me, had me addicted to them, and ones that made me come back multiple times even after completing it such as Doom 1 (Ultimate) and 2, Half-Life 1, Halo 1, Turok 1 and 2 and a few others. Additionally, the DLCs are great, especially Tiny Tina's Assault on Dragon Keep. That DLC is easily one of the best DLCs I've ever had the pleasure to buy and play. It's actually more of an expansion pack than anything else. It's big, full of content in its own region of the game, has a very fun and refreshing spin-off story to it and the writing is absolutely fantastic for Tiny Tina, not to mention that the voice acting is impeccable coming from Ashley Burch (and the voice acting in general, quality-wise is like that throughout the entire game too).
Anyway, I could keep on going about it. But I'll conclude by saying that I finished the first Borderlands as well, once, and never came back to it after. It was fun overall now don't get me wrong. I genuinely liked it. But by the end it was losing a lot of steam (for me) and the last fight itself was extremely underwhelming, anti climactic and frankly bad enough to insult my intelligence even if I tried to take it with a grain of salt. But that's just me I guess. Fast forward to Pre-Sequel, hesitated for many months before eventually buying it pretty cheap during a sales on Steam. I absolutely tried to play it past the 3 or 4 hours mark of playtime during any given session, and I can't. I don't "hate" it, it has its positives. I thought that the 'intro' was fun, and from the amount I did play past that point I'm starting to like the 'descent' in mental stability of... well I won't say too much in case some people haven't played BL2 first. What I do dislike about Pre-Sequel for now is the itemization on the Normal setting; compared to BL2's it's horrendous, weapons are plain and simple boring and the loot pool is just bad in general, and that's saying this while I do think that BL2's Normal setting isn't exactly amazing (but it SHINES like a hyper giant star in True Vault Hunter mode). Also, the oxygen ("Oz Kit") system when outdoors was simply not necessary at all, no matter how hard I try to cut it and accept it as an extra "gameplay mechanic". It's not, it's simply very annoying even if it's sort of generally easy to manage; still don't like it, still wasn't necessary. I managed to finish BL1, but I couldn't be bothered to come back to Pre-Sequel, plus Pre-Sequel's characters - overall - simply don't interest me much (except Claptrap, even though his skills and playstyle is pretty much all over the place and chaotic; not my cup of tea but I can see it's original enough).
I'm also happy knowing that BL3 is being developed. But I honestly don't expect it to ever top BL2.