On the whole, X3 OST...very hit or miss; some are great, others are boring (Tunnel Rhino, Blast Hornet, Neon Tiger, Volt Catfish, Kaiser Sigma) It has tracks I like (they all have tracks I like) but it's closer to the mid-bottom for me out of the main 8 titles. Ultimately outdone by others, because X Series is largely great for music.
X5 and X1: too close to call for me...Okay, X5.
X4: very upbeat. So happy Opening Stage X was chosen for X's theme in Project X Zone.
X2: it mostly sounds very good, it feels very tailored to each of its own stages. Only one I don't like is Agile stage
X6: mostly good, bleak but energized, but I think has some phenomenal stinkers too: Metal Shark, Menu musics
X3: almost half good and bad, doesn't help that there's no prevailing mood. Not much better than
X8: it's okay, almost half good (Inferno, Central White, Troia Mission) and half bad (Mines,
Dynasty, Sigma Palace) like X3. blander compared to other games. Except Dynasty REALLY sucks. It's way ahead of...
X7: too many bad songs, most lacking in impact overall.
Apparently the guy making Mega Man X Corrupted considers X3 the best SNES game. I don't see it, though he is going for the customizability that X3 had somewhat.