I would query all the agents that you feel stand the best chance at selling the type of novel you have, the ones that fit your style and have recently sold (within the last year) what you write.
I'd be hesitant about sending queries to the "lower agents" first, because if they bite, then you have no queries out at the agents you really want. While it's perfectly acceptable to contact agents currently reading your work when you get an offer, it's a bit unprofessinal to send out a new round of queries after you find out your query "works." Make sure the query works before you send any out.
I worked from my top choices down. I had about 8 agents on my A-list that I researched and felt I was a good fit for. I queried all of them. I ended up signing with one, but had I not, I had another round of 8 or so on the B-list, and then my C-list.
One thing to remember about queries...
A fresh idea, strong voice and proof that you can write will get you requests for pages. It isn't about the right format or the perfect combination of things. If you have a great idea agents haven't seen a million times, your query proives you have command of the English language, and you come across as a professional, then you have a good shot.
Having a query that is sloppy, unprofessional and badly written will sink you every time, even if the idea is good. You'd be shocked at how accurately you can tell what a novel will be like from the query.
Good luck!