I have an FD2, OCD, and an OD808. I like the FD2 and the OCD the most. (I replaced the 808 on my board with the FD2 for what they do in and of themselves.) Of those two, it's a tough call: The FD2 makes for more cut, but at a loss of some low-end. The OCD retains more of the lows, makes for a fatter boost, but doesn't cut as much. However, that pretty much describes how those pedals boost any gain stage, be it the Riot or the amp's own dirt. I think I might be reasonably safe in saying that whatever pedal you like for goosing an amp's distortion will probably suit the Riot as well.
I have both the FD2 and OCD in front of the Riot, but if I could only have one it would depend on my guitar playing duties: Two guitar team? I'd probably opt for the cut of the FD2. One guitar player, which I am now, I'd take the OCD. My experience with other boosts is pretty limited, so take that recommendation as you will.