If you're just getting started with ARM MCU's, one of the low-cost and easy to use Freedom boards will be the best place to start: http://freescale.com/freedom (FRDM-KL46Z being the most feature rich of the Kinetis L families currently)
I'll just say the 'obvious' answer -- The Freescale Kinetis Family! In their broad range, I am sure you will find a 'good fit' for each performance/cost/integration/function/power-budget/... you might be looking for!
