Symbian. It's a dying platform but I'm pretty happy with it. It was the cheapest smart phone my carrier (Straight Talk) offered at the time.
Blackberry OS won't be around much longer so I dont suggst going with a dying platform. About the only difference anymore are the apps, but I'd chose Android over iOS any day due to being able to have custom roms etc.
I previously used ios, but now android ftw I always wanna try out blackberry os but just not gonna do that, in short blackberry os is least interesting to me.