I recommend IAR and CodeVision.
CodeVision: It's easier to use and a lot of examples and configuration wizard for beginner. For large project, IAR should be the right choice, I think.
Anyway: AVR architecture is the best over other 8-bit microcontrollers, because of unique technology such as
1. Number of system clock cycle : 1
2. Wide operation voltage range without using built-in regulator: (1.8 - 5) V
3. Easy flash programming via LPT port
and so on.....