At last! An easy one to answer. My favorite film sequel is hands down X2: X-Men United (2003). The original X-Men (2000) is splendid, but by necessity, it has to spend a lot of time introducing characters, giving you a taste of their back stories, and explaining what their powers are. They also have to explain about mutants, the human- mutant conflict, and so on. I have been known to watch a shortened version of X-Men by fast-forwarding through the plot points and watching, I admit it, just the Wolverine (Hugh Jackman) parts. Because long before the movies came out, Wolverine was my favorite X-man. Far and away my favorite.
But I never fast-forward through parts of X2. It is magnificently focused, the characters all straining toward a common goal: not getting annihilated by William Stryker (a pitch-perfect Brian Cox), who once gave Wolverine his adamantium skeleton and is now on a mission to kill all the mutants in the world. So instead of fighting other mutants to save the humans, our X-Men are now joining forces with their usual enemies to save each other. And because most of the main characters were introduced in the first movie, this one gets to spend exploring relationships and letting characters grow and change.
And, because of his former relationship with Stryker, this film naturally focuses a lot on Wolverine and his forgotten past. Obviously, I love that. Really, really love that. In the first movie, Wolvie is in his surly, bad-tempered mode around almost everyone except Rogue. In this one, he gets to show his quality more, starting with protecting the students at Xavier's School for Gifted Youngsters during a scary night attack on the school. That part has one of my favorite quotes, which he's saying in this screencap: "If you're gonna shoot me, shoot me!!!"
It also has this lovely moment with him on one side of an ice wall and Stryker on the other, with Wolvie just starting to remember how they might be connected. The movie has quite a few nice visuals like this -- I think it's the most visually interesting of all the X-Men movies so far (though I haven't seen First Class yet, though I'm hoping to soon).
The only aspect of this movie I don't like is Lady Deathstrike (Kelly Hu).
I'm not a fan of her character in the comic books either, and mostly because she just doesn't make sense. Wolverine's claws could somewhat conceivably be sheathed in his muscle-bound forearms. The length works, at least. But Lady Deathstrike's claws? No way are they sheathed in her fingers! Or even, really, her hands. Just doesn't work for me. Also, Sabretooth had the my-claws-grow-at-will superpower going on long before she came along, so she's kind of boring. But I try to ignore her.
So, yeah, I said this was an easy question to answer. I didn't say the answer would be short ;-)