Probably Slide Away or Cast No Shadow. Although they have a ton of great songs that I love. Acquiesce, Hindu Times, D'you Know What I Mean, Supersonic, Rock & Roll Star...heck pretty much the entire first album.
Honestly, anytime I hear Noel's or Liam's voice the song has my backing seal as a great tune. I like them all. here are some of my faves
Cigs and Alcohol
Roll With It
D'Yer Know What I Mean
Don't Go Away
Sunday Morning Call
Force of Nature
Little By Little
and anything off of Don't believe the Truth and Dig Out Your Soul and and any B-sides from those eras that I have heard ( Lord Don't Slow Me Down gets me any time I hear it)
It is really hard to choose, especially from the later albums. I feel like the addition of Gem and Andy really gave the band a boost to put out stuff that rivaled the first two albums and the great tracks off of the middle albums. Almost an injustice that they broke up when they did, but now We have solo Noel and Beady Eye! Though, I still feel that we haven't heard the last of Oasis!