Goodbye Esaka from King of Fighters 2000. As someone with a serious weakness for guitars, I had a massive eargasm the first time I heard it.
The King of Fighters 2000 - Goodbye Esaka (Arranged) - YouTube

I've always been fond of the Electric Fountain theme from Tekken 6.
Tekken 6 Soundtrack: Electric Fountain - YouTube

And Charlie's theme from Street Fighter Alpha never fails to fill me with nostalgia.
Street Fighter Alpha - Charlie Stage Theme - YouTube