Not counting songs that are extremely short, like NOMAC/intro/interlude tracks. Here's a top 10:
01. Dream Theater - Repentance, easily, no contest.
02. Diablo Swing Orchestra - Hmmm. Probably the back half of Stratosphere Serenade or Aurora. No obvious duds for me.
03. Muse - Psycho for the atrocious lyrics (but it has a decent riff so eh), Guiding Light in general.
04. iamthemorning - K.O.S. or I Came Before the Water Part II, I guess. No obvious duds for me.
05. Taylor Swift - End Game. Most of her songs with guest artists don't really work that well, but End Game is BY FAR the worst.
06. Haken - Veil.
07. Ne Obliviscaris - Pyrrhic or As the Icicles Fall. Not exactly bad, but they don't hit as hard as the rest.
08. Epica - Beyond the Matrix: The Battle (featuring the Metropole Orkest)
09. Ayreon - Web of Lies. Not sure what purposes it serves.
10. Sergei Rachmaninoff - Most of the REALLY early stuff, like the Three Nocturnes.