While these next three suggestions are not my own personal feelings, I do enjoy these first songs from these three artists, and some fans might view them as (one of) their best songs.
IQ - The Last Human Gateway - the sprawling 20-minute epic that opens their first proper album is a classic for neo-prog, and it really set the tone for their sound. I haven't spun this album in awhile, but I recall always enjoying it as much as any of their later material.
The Tangent - In Darkest Dreams - another 20-minute epic from the band's first album, which only had 4 songs on it anyway. This is one piece even the band continued to play for years after the album's release, and is a classic Tangent piece, but for me, it's merely just ONE of the band's best songs. The Tangent have gone through MANY changes, so to really compare this epic to their later ones would be a bit unfair, but to me, they're ALL great songs.
Transatlantic - All Of The Above - the mostly-Neal-penned epic from the supergroup's debut album is a landmark in the world of modern prog. How do you open an album that featured members from four of prog's greatest bands? Why, with a 30-minute epic of course!!! It's got everything that makes symphonic prog such a fun thing to listen to, along with some great playing from every member, this is easily one of my favorite epics ever, and I'm sure plenty of TA fans will cite it as their favorite above all the other TA pieces, but for me, I enjoy their later albums just as much as the debut (if not a tiny bit more), so while AOTA is great, it's just as great as what was to come.
-Marc.