Hi. I'm a bit of an o/d pedal nut, and I've got a klon, a providence sov-2, a tubescreamer (maxon version, od9) and a carl martin hot drive/boost. The one to go for depends on what you want it to do - the klon has the best "clean" boost sound and will make an already good sounding guitar/amp combination sound bigger and fatter. It can sound a bit too mid rangey with some humbuckers if you're not careful. The tubescreamer will always sound like a tubescreamer, it's great if you want that srv sound, but it's always recognisable as a tubescreamer. My current fave is the providence - awesome sound, very smooth sounding, the clean boost almost as good as the klon. Only got it a couple of days ago, so it hasn't been gigged yet but so far has sounded good with every guitar I've tried it with. It's used by robben ford and matt schofield.
Hope thats helpful!