Isn't the Cobourg courthouse still in that nondescript 50s building out close to the 401?
The nondescript 50s building might have a courthouse (I guess this must be the Northumberland County offices as well), but I am sure notyouagain is referring to the exquisite Victoria Hall, which still has a functioning courthouse as well as a theatre and council chambers.
Kingston has a wonderful courthouse, as does Peterborough, Woodstock, London (the original, which is now Middlesex County offices), Stratford. I also like the later Huron County Courthouse in Goderich, and the small courthouses, like the original Brampton Courthouse (which is one of very few legal county complexes that remains intact, with courthouse, registry office and county jail complete - Woodstock and Napanee being the only other examples I can think of), and Lindsay (now municipal offices).
Unfortunate losses include Hamilton's Wentworth County Courts (demolished around 1960), Belleville, Barrie, Windsor.
Woodstock has arguably the best intact collection of county-seat level civic buildings, including a town hall, a Dominion Building, a Carnegie Library (which remains in its original purpose), a jail (now municipal offices), a registry office, a courthouse. Plus the railway station and some old railside factories. Brampton has all of these as well (the town hall there moved around, but two of its former locations still stand), but I prefer Woodstock's collection.