Not so much a ton of manufacturers as a lot of brands.
For "traditional" pop-ups with tent ends, since Fleetwood left the towables market, the manufacturers come down to Forest River (Rockwood/Flagstaff), Coachmen (Clipper/Viking), Vanguard (Palomino), and Jayco/Starcraft. Since Forest River owns Coachmen and Vanguard, for me that is just two.
If you are looking at used, Fleetwood used to make five or six models, and also made the Coleman brand.
Look at the models available for the brands that are sold in your area, choose a model that fits your needs: size, weight, floorplan, amenities.
I'm shopping popups and folding hard-side (latter category includes F-R, Jayco, A-Liner, Chalet).
I visited the Jayco factory showroom yesterday morning, and came away a bit disappointed. Jayco has greatly pared back their model lineup for 2015, dropping a couple of my top candidates from 2012-2014 model years, and gone to a door that folds down to be a step, which I absolutely don't like and will not buy. Also, on the hardside folder I inspected, I found a few "fit and finish" issues I don't usually expect from Jayco, starting with a poorly fit mitered joint in aluminum trim on the door, exposing my hand to sharp edges and metal slivers.
Right now, for the widest selection of models, look at either Flagstaff or Rockwood, what dealers have and what else is offered. They are twins marketed separately, so a dealer will have one or the other, not both, but it doesn't really matter which one. Both brands have three trim levels in the pop-ups, plus high-wall and hard-sided models.
I'm not sure how F-R's Palomino Basecamp and the Clipper/Viking twins relate, many floorplans are similar to Flagstaff/Rockwood models, but not as many variations are offered.
Tom Test
Itasca Spirit 29B