Although I haven't tried it yet, B. 'Gryphon', a seed-grown cultivar originally from Panamerican Seeds, has been getting a lot of positive attention the last couple of years. It gets rave reviews and I hope to try it in my garden this year. Haven't seen it in any nurseries or garden centers yet but seed of 'Gryphon' is now available from many sources so it's probably just a matter of time.

Funny thing, Burpee is now offering an "exclusive" Begonia cultivar they are calling 'Tropical Nites'--funny because their "exclusive" plant appears to be the exact same thing as 'Gryphon' (they even use the same promotional photo!). I would say this goes beyond "funny" into "shady" and possibly even "dishonest". It's "exclusive" only in the sense that they're the only ones selling 'Gryphon' under another name.

PanAmericanSeed's Begonia 'Gryphon': http://www.panamseed.com/plant_info.aspx?phid=009205186024979

Here is a link that might be useful: Burpee's Begonia 'Tropical Nites'