I just wanna pop up from silence to slap ya for mentioning MIUI (joking here ). We're talking customization and open-ness here and MIUI keeps too much things to itself (theme takes real money, services too localized for Chinese...)
I'm a Chinese and lives in China myself, but despite MIUI's saying it's "the best OS for Chinese", I still find myself sticking to CM on most occasions because they are totally open and updates FAST!
==========Off-Topic above===========
Having looked through all the suggestions that had been made, I'd say that somebody will regret picking up a sizable phone like a Galaxy Mega... I recently awarded myself a Nexus 4, but just 4.7 inches of screen is enough to make the phone wobbly in hands when I browse the web and text with two hands, let alone single hand operations. And Samsung turns people off with its endless variations of one model (each gets minimum support, you are warned) and plastic shells. So from what's available now, picking up an HTC New One (Google Edition optional) or Xperia Z/ZU could be the flagship choice.
And mind sharing what's actually available for ya? I reckon not all flagships we're talking about are feasible for your plans...