since i'm an avid lover of nobuo uematsu and final fantasy, i'm going to have to list my absolute favorites. starting with the song that makes the game. every last one of them. but i'm going with this version as my favorite:
https://www.youtube.com/watch?v=BvMVi65XVAQ
moving backward to ffx, i can think of two songs worthy of note. the chilling theme of Auron, one of the most kickass main characters in the series..
https://www.youtube.com/watch?v=8pCJbp5dfY0
and the recurring theme of the game, To Zanarkand. i don't know if it's one of those acquired tastes since i heard it at least 50,000 times during my playthroughs, or because it just stuck with me, but i found it moving.
https://www.youtube.com/watch?v=h-0G_FI61a8
then if i had to pick one from 9, it'd be Eternal Harvest.
https://www.youtube.com/watch?v=kcoMMAlopMU
though Loss of Me/Rose of May 
https://www.youtube.com/watch?v=l4bnYF2m8eQ 
and Freya's Theme 
https://www.youtube.com/watch?v=b2rlLC4N9u4
are a close second and third. ...possibly in that order.
i'd also mention you are not alone, but it's already been mentioned~
since i can't decide between my two favorite tracks of 8, i'll put them in order of appearance:
The Landing - if ever there was a song that screamed wartime in final fantasy, it was this. 
https://www.youtube.com/watch?v=UMizBfC9Yi8
and second, The Oath. another theme that just feels..inspirational. 
https://www.youtube.com/watch?v=sr4eB0WOwN0
for everyone's favorite game, or at least the most famous, 7, i have to go with the most ominous, the most inspirational, and well. possibly one of the most famous of the soundtrack. in order:
Those Chosen by the Planet: 
https://www.youtube.com/watch?v=3rV40ypje7o
Cid Highwind's (kickass) Theme: 
https://www.youtube.com/watch?v=SJzfOA9uV6E
and of course.. Aerith's theme: 
https://www.youtube.com/watch?v=fIqKWLkm2-g
and now for my personal favorite game of the series thus far, numbah six.
...you know what on second thought, there are so many tracks in six that i love i'll have to do it in another post. so we'll come back to that~
so 5! let me start right off with one i've been looking forward to on this list, possibly the best battle theme in the series. 
Clash on the Big Bridge: 
https://www.youtube.com/watch?v=6CMTXyExkeI
then there's Faraway Hometown: 
https://www.youtube.com/watch?v=U2a1iigJoXs, which for me is just a song i could listen to over and over again without getting tired of it. it just has that repeat value.
moving on to four, there are two favorites i have that are definitely worth mentioning.
firstly, the ORIGINAL chocobo theme from which all other chocobo remixes sprung forth: 
https://www.youtube.com/watch?v=598tWx5tALQ
and secondly, the eerie, creepy, song about walking talking children's dolls, Dancing Calbrena: 
https://www.youtube.com/watch?v=tnY9XMYu-1s
skipping over 3 and 2 as one i had no favorites from in the music and the other i've honestly never played, the original final fantasy has three tracks worth mentioning:
Cornelia Castle: 
https://www.youtube.com/watch?v=qvEodqaRHvQ just overall nostalgia factor.
Chaos Temple: 
https://www.youtube.com/watch?v=suyCi-xdjNo why is the first boss of the game the owner of a track that should belong to the last boss of the game?
and Matoya's Cave: 
https://www.youtube.com/watch?v=sUc_t3i4qwE because it's just so upbeat!
finally, i'm sorry for the epic post. but i figured it was better than posting each song seperately xD