For some that's one cell, so it would be the RRT-0. Others accept bigger, so it could be the RRT-2 or even the RRT-1.
Those having an Olight SR90 say the RRT-3 is an optimal EDC...
Jokes aside, I'd go for the RRT-0 and if you preffer the IBS UI you get the Jet-I-Pro V3.0 even a little cheaper, the difference in brightness is not that important.