There isn't a particular song that sticks out, I enjoy the majority of their work, even if they have gone through some stylistic changes, I enjoy both their rock and classical music...however: The following songs in my opinion do not really 'catch up' to the rest of their work...
Forever Love: Generally I love their ballads, Endless Rain and Tears being my favorite...but this one...I don't feel it like I feel their other ballads.
Easy Fight Rambling: You'll still find it in my iTunes...but it just sticks out with its "happy" sound, so it doesn't feel very "X Japan".
Alive: Can't really explain this one...its just me and my personal feeling that can't explain it ^^;
However, I do want to comment on other opinions:
There has been alot of talk and debate about Taiji's contribution to X. Even though Taiji did write many songs for X, many of them never made it on the albums. The stuff that does appear, I very much enjoy. Phantom Of Guilt is pure awesome! Desperate Angel, even though it does sound different (personally this song always make me think 'cowboy metal' for some reason, haha) I listen to it as much as I listen to Rusty Nail or Kurenai. XClamation you can't even touch, its an awesome song!
As for the ballads, I love them (with the exception of Forever Love). I can see why you would not like them as opposed to their rock songs. Without You is a great song, and I would love to see it performed live, as with Tears and Endless Rain.
And finally, their new work. I think I.V. is an awesome song, and is performed wonderfully live! Jade is also a great song, works with the progressive sound of Rusty Nail and Dahlia. Born To Be Free however...I have very mixed feelings about it...it just doesn't identity itself as an X Japan song to me.