What do you consider better, or more to the point, what can't you do with the M128?
The best microcontroller, is the one that fits the task. A Tiny13 would be a better chip, if you just want to flash a few LEDs, or read a sensor and transmit the data serially. Haven't had need of a Mega series yet, so not sure what it can and can't do.
I'm using the mega128 for quite a long time, it's one of the best ones anybody can have!! what problem you faced with that one??
Specify the parameter you're searching for in the 'BETTER' microcontroller..