Depends on what you're looking for.
My two favorite RM bags are my black MAM and my seafoam mini Nikki. If I could only have one, I'd choose the black MAM (i.e. the perfect black bag). It has a very classic, ladylike shape that dresses up or down easily. It's not what I would consider a "fancy" bag, so I wouldn't wear it with evening clothes, for example, but it looks refined with a wrap dress and heels. However, because it slouches, it just as easily looks casual with jeans. The mini Nikki has a much more casual shape, so it's less versatile IMO. What I like the mini Nikki for is a fun pop of color. It's not as "serious."
Both are a good size and comfortably hold everything that I need on a day-to-day basis (and I carry a fair bit).