RubyMine is a commercial IDE for Ruby built on JetBrains' IntelliJ IDEA platform.
It is at the time of this writing the best environment to work with RubyMotion. RubyMine features context-based auto-completion, point-and-click debugging (on both the simulator and the device), inline documentation, and so on.
Please refer to the Getting Started with RubyMotion guide on JetBrains' developer website for more information.
3. Sublime Text
The builtin rake ctags task can be used to generate a tags file containing all the APIs the project can call into. This file can then be used to automatically complete API calls in the editor. RubyMotion uses Exuberant Ctags. Ctags is a format understood by a variety of text editors, Vim included.