I don't have four spare R2's or R3's so if I take them from the other parts of the board, it's a bit hard to test how they feel during "unconscious" usage, i.e. typing naturally without thinking about it, because the other parts of the board will have weird keys. But since the R1's are acceptable, the R3's are closest to the slope of the sphericals (i.e., level), and the R2 is in between, I'd guess my order of preference is R3, R2, R1 with R3 being the most preferred.
But when I get home from uni today, I'll swap some R3's from the other side of the board and see how "unconscious" I can make it :-)