The Odyssey - As epic as it gets IMO, and maybe my favorite metal song.
Awakenings - A beautiful blend of heavy and soft, without being erratic, like a bourbon with a good bit of rye.
The Divine Wings of Tragedy - Given the choral intro this one is every bit as epic as the odyssey, and I love the part they took from mars, plus this song had a million great melodies, and is a great example of their musicianship (time signature shifts and whatnot). Btw if you are a metalhead and/or really like movie music, get the planets by Holst. It's amazing how much came from that.
Through the Looking Glass - One of the most interesting stories Symphony X has done a song on, and that's saying something. Excellent vocals, and overall a great song.
The Edge of Forever - Read the above comments and them consider that I had a very hard time putting any song above it. Great chorus, that's what they, and all epic power metal, do best.
The Sacrifice - My Favorite from Paradise Lost, which was an absolutely perfect album. The odyssey, Dwot, and V were as well, but PL was just simply the best.
The Accolade - A great song, especially melodically, a great story, and makes the second one just that much better.
Paradise Lost - Hard to pick a best song from that album, but the title track is in the 8 spot.
Accolade II - Nearly as good as the first, and great musically and melodically.
Revelation - It was hard to put this at number ten, but it's just amazing. Awesome way to close out the album
I can't resist another ten:
Communion and the Oracle - Excellent song, hard to leave off the top ten, and maybe Romeo's best solo.
Eve of Seduction - Musically impressive and one of the best choruses ever (though I think the chorus to The Sacrifice is just a bit better). Another hard one to leave out.
The Walls of Babylon - A great epic track to lead into another great epic track.
Candlelight Fantasia - Muscally and lyrically beautiful, and very meaningful to me as a composer.
Fallen - Love the chorus, nice and heavy.
Evolution - Another great song from V, which is basically a perfect album. I love the way it plays as a whole.
Lady of the Snow - One of their most musically and lyrically interesting songs, I like that it's not a well known story on the level that the Odyssey, Through the looking glass and DWOT/PL are on.
A Winters Dream (I & II) - Very well put together, and really the only better song on that album is Edge of Forever.
Seven - Another perfect song from a perfect album, great intro... Great everything.
Wicked - I could have put this higher, but I burned out on it from listening to it too much. The Odyssey introduced me to Symphony X a few years ago, and they have since become my favorite metal band. I could pick 40 perfect Symphony X songs. Their catalogue is the best in metal IMO. When I'm in the mood for epic music, it's this, Howard Shore, Holst, Zimmer, or Williams.