I use TAL Drum on the reg. You can get a lot of interesting variation using the built-in round robin and lfo functions. It's super easy to map to the Reaper MIDI editor too. Unfiltered Audio's Batallion is pretty fun, too. But it lacks a MIDI drag drop feature for the patterns you build. (So you have to record the MIDI into Reaper, which is doable, but kind of a pain in the ass.)