1) AR-platform -- I hated them...until I tried one. Now I have 7 of different variations. I love the modularity, ability to create what you want, variations.
2) XD -- I didn't like plastic...and my experience with Glocks confirmed that. However, it ended up being that I didn't like the grip angle of the Glock. My XD eats anything, shoots like a dream. It would be my go to fighting handgun, even though my sentiments are with the 1911.
3) AK -- I thought I hated them, so I bought one to confirm. I still don't love them but I appreciate what they can do. I have 3 variants -- AKM, Galil, Saiga-12. I prefer other rifles over them, however.
4) HK91/G3 -- thought I loved them...then I bought one. Ergos are horrible and they are "clunky"....reliable and tough but not refined like a FAL or AR. I own a PTR-91 and a CETME, but I wouldn't choose them in a bind...I wouldn't refuse one but I prefer the FAL or AR.