The preferred one is the one you feel more comfortable with, and that achieves your performance objectives.
Propeller performs well, but has much weaker resources on the web if you don't know the particulars of whatever you're trying to do.
Atmel AVR8 (Arduino Uno and friends) has a little less raw performance, but is a robust chip and there are tons of tutorials on the web for anything you'll want to do.
ARM 7 (Raspberry Pi) is another option, it has a lot of performance (for things like image processing etc) and some amount of digital I/O, but doesn't have as many timers/PWM/peripherals as the Arduino. Also, it is 3.3V (like the Arduino Due) instead of 5.0V (like the Arduino) for I/O.