There are so many songs I love, it's pretty impossible to make an absolute top 10, but here's 10 tracks that would at least be considered for such an impossible list

In no particular order:

Ar Tonelico - Chronicle Key

Ar nosurge - em-pyei-n vari-fen jang (RIP Origa )

Ar Tonelico 2 - EXEC_with.METHOD_METAFALICA/. (I'd just say "Metafalica" but there are multiple versions so the distinction's important )

Ar Tonelico 2 - Sublimation ~ omness chs ciel sos infel (Last Ar Tonelico/nosurge song, I promise)

Ar Tonelico 3 - Hymmeli (okay I lied, last one for real this time)

Trails in the Sky the 3rd - Maybe it was fated −Instrumental Ver.− (although it's initially an arrangement version of a Trails in the Sky SC track, this version does play in the third game)

Trails of Cold Steel 4 - Atrocious Raid -Super Arrange Version- (similarly, initially an arrangement version of a Trails of Cold Steel track, but it plays in 4)

Ao no Kiseki - The Azure Arbitrator (possibly my favorite final boss track ever)

Xenoblade Chronicles X - z5m20i12r04a28 (Mira theme) (Yeah, Sawano's track names can be weird. This was a tough pick because I knew I wanted Xenoblade X on this list, but I love Sawano's music in general so it's really hard to pick just one track from the game. By the way, this OST gets a bad rap because of a few tracks that you hear a LOT in-game, but the OST overall is amazing, go listen to it )

Yakuza 6 - Destiny (Yakuza has some amazing boss themes, this is probably my favorite one)

Honestly I could easily think of another 1000 tracks that deserve a spot here.