Clean boosts seem to be much better in getting more drive from an existing good sounding amp than any OD I've heard so I like 'em. I've been using a Banzai New Rising Sun II for boost since a few years and it's nice. There are two footswitchable boosts with separate volumes and a switchable buffer amp which is nice to work with. When I got it I tested it against a Zvex SHO, Xotic AC and RC boosters, the Fatboost and a Crowther Hotcake. The Banzai sounded most natural to me, ie full range boost without colouration. If you don't need any tonal options this would work well for higher gain styles.
I wouldn't get the Zvex SHO because it adds alot of sparkle which is nice for '60ies pop but not so for brutal metal/thrash. I'd suggest trying the Xotic BB preamp because it offers a broad tonal range and huge clean gain boost. It can also add some extra drive with that if you prefer that. The RC and AC are also nice but a bit mellower.