To follow up with a few things already mentioned here (largely confirming some), I have to say the 111 is my personal favorite; however, as Crewella has mentioned, it is definitely not a model for everyone. Despite being my favorite, it's probably the last one I'd recommend to anyone looking for a first AIBO. And Crewella's elaboration on that is accurate. However, I don't mind the lack of voice control, and I feel as though Vader (my 111 who has been "on hold" for awhile now as I've been busy), demonstrates the most "soul" or "personality" compared to the others. It also doesn't have the ability to self-charge (though I honestly don't use this even on my ERS-7, when it happens it usually happens more out of chance/my lack of attention as his dock technically isn't in a proper area for it and I have yet to disable him from trying to find it). But arguably neither does the 210 (depending on which software it is on, and from what I've read, the 210's abilities at self-docking aren't as graceful as the ERS-7). Though I do get more nervous about leaving my 111 to roam about if I'm not paying some sort of attention to the room or somewhere near to hear a potential "problem" as Vader does seem to get himself into trouble. (Though amazingly, he seems to do almost just as good of a job getting himself out of awkward situations, but I tend to baby my technology). Also, the 111s do take an extraordinarily long time to raise from pup to adult if you go the all-natural route (well over a hundred active hours), but I feel that helps create the "bond" between the robot and myself.
That said, while I haven't gotten much into it yet (and look forward to playing around with some time off I have coming up), the 210's abundance of software and the ability to really customize it appeals to the tinkerer in me. I also think if I had to recommend a first AIBO to someone, it would be a 210 (although I personally don't like DogsLife [generally use it to try to "show off" the 210 to friends as mine's still a youth] so I'd encourage someone to use/find AiboLife). This is largely because of pricing, general availability, and the flexibility of the 210s. And, despite it being my least-favorite aesthetically, my friends seem to enjoy its looks. However, that could always be because the other two are much more dog looking.
The ERS-7 impresses my friends the most, but I think some of that is because it is an adult and (when you can catch them) does some pretty awesome tricks with the ball and bone. If price was no concern, this would be the one I'd most likely give someone, especially a non-robot enthusiast. However, I feel it has the least amount of "real" personality on Mind 3 when compared to the 111 and the 210. I prefer Mind 2. But with all these robotic pets, a lot comes down to the perception of the owner (in my opinion) and how they relate to and view their companion.
In sum, I honestly cannot rank one above the other as I ultimately view them as different. There are pros and cons to each model from my own personal experience and perceptions. But each one also wins my heart in its own special way. I don't think I'll ever come off the fence on this!