Which IDE should I use for Vala?

I think the title explains it already...

As @aperson said, Geany is a very good text editor - It is lightweight with lots of features. It also supports vala (you need to install valac first though for full vala features). A lot of its features are IDE like eg. you can build/run with 1 click.

To install, run sudo apt-get install valac geany or search for 'valac' and 'geany' in Ubuntu Software Centre.

There is new project called Valama, you can check:

https://github.com/Valama/valama

It uses gtksourceview, so editor experience is similar to gedit, but it's still in early development phase.

UPDATE: It is getting better every day, there is active development on going.

There are two plugins for Gedit that provide Vala support. Valencia and VTG both add autocompletion, symbol browsing and basic project management through makefiles

Valencia is the easier of the two to setup because VTG depends on gtksourcecompletion, but VTG has made several recent releases.

http://yorba.org/valencia/ http://code.google.com/p/vtg/

Anjuta supports vala since ver. 2.31.3 and there's a nice plugin for gedit. http://redmine.yorba.org/projects/valencia/wiki

I can't recommend an IDE specifically, but I can recommend Geany as a great text editor.