I've gone through about 4 or 5 "favorite" kits in the year or so I've been turning.
First Favorite: Euro Ballpoint - Like a slimline, but with a bit more style (slims with style take a bit more thought/effort than I had in the beginning, and maybe still...)
Second Favorite: Sketch Pencil - I have a lot of artist type friends, and this is such a unique writing instrument that everyone I know wanted one. Real easy to make, and a substantial feeling piece in the end...
Third favorite: Zen - I liked these from the beginning, but I didn't order anything from CSUSA for a while in the beginning.
Fourth Favorite: Wall St II/Sierra - Nice look/style, great variety of kit styles/finishes, and a quick, single barrel design makes it easy to turn, and only using 1/2 a blank, makes it great for making matched sets.
Four Point Five Favortie (My wife's at the same time): Atlas/Polaris/Carraba - Same simple single tube turning, nice slightly smaller overall length, slightly bulkier body, so it feels good in larger hands (and you can get one of these out of the leftover blank from the Siera).
Current favorite: Tie between Jr. Gent and Navigator/Barrons - I REALLY like just about everything about the Jr. Gent, except having to mail order them. I don't tend to plan that far ahead. So I've only made a few of them. I have a local Woodcraft store, which makes buying one or 2 navigators at a time very easy, cutting down on having to drop $50-100 to make shipping worth while. They also started carying the closed end mandrels for them, greatly expanding the flexibility of the kit.
As for materials, it has changed just as much. In the beginning, I had a bunch of Ipe left over from my deck, so I did one pen of each style out of that first, which was great, since I made a LOT of mistakes (buy extra tubes is my new rule of thumb..) I also used a bunch of my firewood, and some Home Depo variety oak scraps left over from framing all the windows in my house. Early on Free ruled... Went through a cool colors phase, bloodwood, blackwood, bubinga, purpleheart, lignum, holly, and buckeyburl. The buckey turned me on to anything stabilized, which turned me on to worthless wood. A local chapter meeting at Bad Dog burls turned me on to so many of the Aussie burls, as well as Desert Ironwood Burl. Around the time I got into Stabilized woods, acrylics ran along parallel and are always present...
My suggestion, try something new every so often, and see where life takes you.