Absolute worst for me: PRO-94. What a dog brucellosis-ridden, gut-shot feral hog.
Its hard to name one favorite, but the one with the most miles on it is a PRO-2026 that was in my truck for over a decade, and serves as a railroad scanner today. My PRO-2042 sees a lot of service scanning MilAir and VHF today.
Out of the new scanners I've bought, my favorite is the BCD325P2. With ProScan, it just takes a few minutes to load it up with whatever I'm going to be scanning that day. I get 5-6 hours from a set of Ni-MH AAs, so I don't need to lug a bunch around (although I do because I lug around multiple radios when I'm out listening).
Favorite form factor? The good ol' kitchen counter scanners. My BC365CRS runs almost continuously, and helped me find some unlisted business freqs. My old BC855XLT still works great, and is a veteran of many hunting and fishing trips around Texas. After a long day on the water or in the woods, it's a nice activity to go along with good food and old Bourbon.