My preference is unmelted butter. I like it put on like cake frosting.TR likes his toast with a mix of melty and solid butter.
Nigella Lawson has recommended buttering toast twice - once while it’s warm (to get the melty) and once again after it sits a bit (to get the creamy). It’s nice, but I like it thick and solid (there’s “I like my women…” joke in there somewhere).