1, By far Nokia Lumia 1520.
Shame those windows apps are so poor.
.
Blame those trash developers who didn't want to bring those apps to the platform. I still got my bumblebee 1520 around here in a drawer. That thing was built like a tank, that camera was

, always quick and snappy. Now the shell of Nokia is whoring for Google.
Windows Mobile, BB10, and WebOS are the best mobile OS I've ever used but with that being said, to answer the question.
1.T-Mobile Sidekick - I went all over Birmngham looking for this joint and it was out of stock everywhere. I still remember when the T-Mobile in West End called me like "we got the Sidekick in stock and we are going to hold it for you."

If yall only knew...
2. Blackberry Pearl, Curve, 8800, and Bold 9900 - During that stretch of 5 years (2005 to 2010), I used to be in some serious work on BBM..lol. Quietly, I miss the days when RIM ran the game. Look at this 2 "choice" BS we got now...

3. T-Mobile G1 - The OG US Android Phone. I copped this joint on day one to see what it was about and for a few years, it was all about being a rebel.

Til it's market share blew up and instead of fighting to not be iPhone and being the underdog, it became the other side of the same coin.