What do you mean by collapsible? Are you looking for a telescoping fly rod for backpacking? Or are you simply looking for a rod that isn't a single piece?
If you are looking for a telescoping rod then I will be no help at all. I am not a huge fan of them. I have never found one that had the action that I liked. I would rather lug my 2 piece 5wt on a trail with me. When it comes to "take down"style rods that depends a lot on personal preference. In my experience the more pieces a rod breaks into the more feeling you loose. However, 4 piece rods take up a lot less room than 2 piece rods. I prefer light to medium rods with a lot of sensitivity, so I tend to go for 2 piece rods. However if you are wanting to travel with them (flying or a road trip in a small car) then that might not be feasible. I would figure out what matters most to you, sensitivity and feel or compact packaging. Then go to your favorite outdoor store and start playing with rods.