Call me crazy but my favorite smooth dark malt is good old fashioned Briess Chocolate malt. I also enjoy chocolate wheat and chocolate rye. Munch on a kernel or two of each one, whether at home or even in a shop, to know for sure what you like the best.
Hear, hear - Chocolate Malt (350 L) is my favorite of the dark malts: rich roasted coffee and cocoa flavors, but still very mild (absolutely no astringency) when used @ 1-5%.
Black Patent (500 L) is a close second to Chocolate Malt. Highly roasted character w/only a hint of bitterness; surprisingly mild for such a deeply-kilned malt.
Black Barley Malt (500 L) is my 3rd favorite: very roasty/toasty (a little burnt), but still mild.