Snuff and UA are both pretty good, Snuff being significantly better.
Snuff is one of the best I have read in a long time. Its back to the satire (Yes, I know someone might cut me for that) but I love it for the same reasons I loved Jingo, Thud and Monstrous Regiment (Which I desperately need to re-read). Its more "Look, here is a blatant metaphor for things that are going on/went on in the past, now read it and laugh."
Then again, my opinion is just that, an opinion, in my eyes the Rincewind series (If it can be called that) are the worst books in the Diskworld series, plus I never liked the witches. I guess my opinion comes from loving the Nights Watch beyond belief.
It boils down to this. Do you love Vimes? Because Snuff is the most vimes-focused book he has ever written, even moreso than my favourite (Night watch, which if you can find a version of the radio play is DEFINITELY worth listening to). Some have said that it is not the best and that you can start to see his condition shine through, I certainly agree, but he is still Pratchett.
Not my favourite. Night Watch holds that accolade. Thud, The Truth, Interesting Times, Going Postal, Making Money, Jingo come in that order (if that helps...?). Pratchett is easily my favourite author, followed by Neil Gaiman (Their joint work, Good Omens, falls a close second to my third favourite book, American Gods. Which is just a hint beneath Brave New World.). Man, I should read more.