Of course anything by New Order. The Perfect Kiss, BLT, True Faith, and Blue Monday, especially. The rectangle of godlike synth greatness.
Dissapointed, TT, Haze, Make it Happen by Electronic are also on that list.
God, so many Depeche Mode songs, and every Kraftwerk song. Some Orb.
"Its a Sin", "It's Alright", "West End Girls", "Heart", "Minimal", all by PSB are among the greatest.
There are a few Simple Minds tracks, notably some of the older tracks from Sparkle in the Rain and New Gold Dream.
"Hypnotic Tango" by My Mine.
"Love Can't Turn Around" by Farley "Jackmaster Funk" and Jesse Saunders.
"Papua New Guinea" by Future Sound of London
Of course Human League's "Mirror Man", "Lebabon", etc.
Fischerspooner's "Emerge".
"Let Me Go" and "Temptation" by Heaven 17.
"To Look At You" and "Don't Change" by INXS. Very haunting synth lines.
God, there's so many to choose from.
My favorite synth song(s) of all-time would be a tie between TPK, BLT, True Faith, and Blue Monday, though. No doubt about that.