PLT Scheme is hands down the most powerful programming environment in existence. It's sort of like the McLaren F1...those who know, know. And those who don't mistake it for a Mazda Miata.
I'm getting into scheme for a project I'm working on. Sometimes I'm really impressed by how much can be done with some tight coding. I've done some Arc coding as well, and I think that it could have the edge if it matures and gets some library support.