I almost always agree with Mike, but I'm going to disagree ever so slightly this time<G>...
placing the preamplifier stage close to the ribbon has advantages, I wouldn't rule them out. There are subtle differences between passive and active versions of the same microphone, but I THINK they are due entirely to the difference between preamplifiers... after all they didn't stick a Great River or UA in that microphone case!
However, if you already have preamplifiers that are up to the task then I don't think I'd spend the extra $ for a built in preamplifier.
Now returning to the original question...
I like AEA, Royer, Coles, Shure, and Cloud ribbon microphones. I wish I could afford a couple of each!!!
Of the bunch the AEA and Cloud probably sound closest to each other. The Shure (formerly Crowely and Tripp), Coles and Royer microphones definitely have their own unique sounds. If I was just getting started with ribbon microphones I'd probably start with the Royer 121... they won't sound like an RCA, but they do sound great, and with the asymmetrical placement of the ribbon they are very flexible. One of the Shure models shares this feature.
The Royer SF-12 is another candidate as a good starter - it sounds a lot more like an RCA than it does a Royer, and having the ribbons precisely placed makes stereo recording a breeze.
The AEA ribbons will take you back to the days of RCA, and sometimes that is exactly what you want. The Cloud microphones can do the same trick. I'd love to spend a little quality time with both brands, at the moment it may well come down to price because my limited exposure to them suggests that the differences are subtle.
And in the oddball category - I have a printed ribbon microphone from Fostex that sounds nothing like any of them, but still somehow sounds like a ribbon. It is fantastic for brass, and I've recorded some great vocals with it too. I do not like it on acoustic guitar, sadly. And I much prefer the R-121 on a guitar cabinet, but I could press the Fostex into service there.
So many cool microphones... so little money!!