Well...my favourite region isn't really a region...but it also isn't Sinnoh. The Johto-Kanto landmass is probably my favourite region - there's just so much to do, and the aesthetics of the entire game are just...great, possibly better than Unova in Black (everything in Black's crisper, whereas in HeartGold everything's got smooth edges) - or perhaps Hoenn, but maybe that's just nostalgia.
Sinnoh's definitely a good region, if a little disappointing that there's so little water (obviously to counter Emerald's abundance). Especially in Platinum, where they gave it a bit more character and the towns look less...samey. The Battle Zone in particular is greatly improved in Platinum. The variation in the routes - grass, water, snow, bog etc. is brilliant, and the music is really good too.
Although I wouldn't say its my favourite, Sinnoh definitely gets a 4/5 at least. I'd struggle to choose a favourite region, Sinnoh wouldn't be it, but it'd rank above Unova.