X1 and X3 both need it. X3 needs it a lot more.
X1 should've had those Storm and Cyclops scenes. Magneto and Rogue weren't enough.
Logan, Jean and Scott's conversation in Logan's room was better when it was longer. Logan had that great line about whether or not Scott knew about war.
Storm talking to Rogue would have to be redone (looped) without Halle Berry's just horrid attempt at an accent. Truly, it was just dreadful.
As for X3, well, that was cut so poorly, I'd need pages to get through it and get that explanation down.
But believe it or not, X2 could use a few additions. Jubilee's scene at the museum, the full fight between Deathstrike and Wolverine, and that little snippet where Mystique sees Nightcrawler's file on Stryker's computer.
When I first saw the film, I wondered how Magneto knew that Nightcrawler was working for Stryker. The deleted scene explains it and it only adds a few seconds to the film's length. Did it really need to be cut that badly, Bryan?