Butter on toast all the way, to be applied before toasting, not after like at diners where you put the butter on yourself after it's toasted and it's not very good at all that way.
I do, however, love some jelly/jam on my toast depending on if I want something sweet, and what kind of jelly/jam it is. Preferably something like strawberry or raspberry or any kind of red berry is simply delectable when applied post butter toasting, but the bread must be buttered and toasted first or it kind of ruins the taste I am looking for. Not particulaly fond of apple/grape jelly on my toast though. At least, not in the way I am fond of the aforementioned jelly/jam.
Edit: I just remembered! It was strawberry jam that goes best with buttered toast. That is the best, hands down.
Also, cheese goes quite well with toast too. Cheese is the only time I wouldn't butter my toast, because it would probably be a bit too greasy if I buttered it, then cheese'd it. Colby jack, pepper jack, cheddar, blue, it's all good.