What is the best Android on T-Mobile if best = smallest / most compatible?

[AC Question]

The iPhone se looks good to me, but I'm curious about Android. Really looking for the smallest phone that's likely to work with my Wahoo TICKR.

 

[Rukbat]

If "smallest" includes "fewer features", any 3-1/2" phone will work for phone calls and texts.

Trying to get smallest and current features is like trying to find a 15 cent quarter.