BLG writes: "Japanese role-playing games, or JRPGs, are notorious for difficult boss fights. There is something grand about the villains and enemies in JRPGs that isn’t present in most other genres. The best JRPG bosses have a few things going for them."
Kefka, Luca Blight and Sephirot, in that order for me.
Sorry, but Lavos sucks. It has no personality, no motivation, just some alien that consumes worlds.