This does not really answer your question as to what is 'the best' package - but I find Atmels Avrstudio with Atmel Software Framework very easy to work with.
Porting code to 8 bit micros are really easy if your application doesn't need a ARM - I've done this twice. Xmegas has their quircks, with their interrupt system, DMA and hardware multiplier - their performance is quite surprising.
What I meant to say is; Writing code in ASF enables portabillity between ARM, AVR32 and AVR8 micros.
Oh, and AVR > PIC. 'Mandatory fanboi flamewar invitation'.