It's a mix of brains and heart.
Brains: Do they represent my genre? Do they seem interested in the particular kind of novel I write? Are they with legitimate agencies? Do their clients seem happy with their representation? And, of course--how are their sales?
Sales can mean a lot of things: number of sales, the money involved, the publishers sold to, etc.
Then there's heart: When you read their interviews, Twitter accounts, or blog posts, do they seem like someone you'd want to work with? Can you envision yourself discussing contracts with them? Edits? Brainstorming ideas? Would you feel comfortable having them as your back-up in negotiations? Would you feel at ease with them, or constantly be worried about taking up their time, or constantly on the defensive?
This, of course, depends on the kind of relationship you're after. Are you happy with a completely business-like one, or do you also want someone you can joke around with?
If all those things collide, you've got yourselves a favorite agent. I'd caution against putting too much stock in that, though. Tons of agents who I thought would be a great fit for me and my work form R-ed every book I've sent them; clearly, we're not as good of a fit as I thought. Similarly, I think a lot of authors are a little too lenient with their 'dream agent' requirements--I actually blogged about that once:
http://www.corinneduyvis.net/2012/02/on-selecting-your-dream-agents/
Plus, not all great agents have an online presence. Your best bet is to query those agents who are a good fit on the 'brain' front and leave the rest until later. Having favorite agents won't do you a lick of good if they don't want to work with you.