I'd put him a notch above Adams, personally. HHGTTG held a darker, more sarcastic and biting worldview. Pratchett's wit was somehow gentler - no less sharp, but perhaps more poking and less stabbing. Pratchett books had solid endings (not necessarily happy ones) instead of cliff hangers.
He was also quite a bit more prolific, although I was surprised to learn his daily required word count was only 400. (He once said it doesn't matter if you only write two words in one day, so long as they are the right ones.) I suspect he probably spiked quite far above that on some days.