It would be reasonable to think that Atmel Studio is the best integrated development environment (IDE) to use for AVR development. It is the official supported IDE for AVRs, and supports all of Atmel’s AVR devices, programmers, and debuggers.
There are a couple downsides, however. One is that Atmel Studio requires the Microsoft Windows operating system. Another is that it can only be used to develop software for Atmel devices; consequently, most of what you learn about using Atmel Studio will not apply when branching out to other microcontrollers.
Eclipse, on the other hand, is an open source, cross-platform IDE that can be customized with many different plugins. While one of the primary uses of Eclipse is Java development, Eclipse CDT adds support for C/C++.
Eclipse CDT can be easily configured to use GCC cross compilers and debuggers, making it well suited to embedded software development. Once you learn how to use Eclipse for one device platform, it is very easy to adapt this to others. I believe this to be one of the main advantages of taking the time to learn how to configure and use Eclipse.
This tutorial will take you through the necessary steps to setup Eclipse so that you can start developing software for 8-bit AVR devices.