Samsung Galaxy Light
Affordable phones are getting better by the day. The Samsung Galaxy Light comes with a 1.4 GHz quad-core processor, 1 GB of RAM and Android 4.2.2. It is pushed down by a 4-inch WVGA screen, though.
T-Mobile customers that don't need anything fancy would probably be very pleased with this modest smartphone. It's display is not the best, but the processor and software should keep the device running pretty speedy.