This is going to sound bizarre...but the worst phone I have ever used is probably the BlackBerry 8100 Pearl. I know I'll get flamed for that...since there are tons of very happy Pearl users, and maybe my experience was a fluke...but I promise, if the Pearl had been the only BlackBerry ever made, I would not be on this forum using a BlackBerry today. Weird, but true.
I went from my then favorite phone (SE w810i) to the Pearl, and after an evening of dropped calls, poor reception, awful call quality and loud "skronk, skronk" of the trackball...not to mention the weirdness of the SureType keypad that I could never figure out...I rushed to my then-Cingular store the next morning to return the Pearl for another w810i. I couldn't wait...
I'm glad my first experience didn't deter me from giving BlackBerry another chance. I'm hooked now...and having played with several Pearls since then, I know I must have gotten a rotten handset or something, but I didn't know any better.