My favorites have been the already mentioned Mach 1. But lately two I've really fallen in love with are the Bullrush Drive (Providence SOV2) and the Acer Drive (Altere Kaede) I absolutely love how both sound at the low to med end of the gain range.
They're both voiced similarly but the Acer has a lot more low end response.
My latest honeymoon is with the Guardian OD. (Thorpy Peacekeeper) Just buttoned it up yesterday. It's actually got quite a lot of gain on tap but it's probably the most "transparent" drive I've ever played. Really really versatile low to medium gain drive both as a standalone drive or pushing a dirty amp. I'll post a demo later today.