Choosing an online IDE will be much the same question as choosing a native application: what are you looking to get from it?
There are heaps of articles comparing different IDEs, but quite often, the question is attacked with a certain bias. Some developers value simplicity; some value turnkey solutions; some value extensibility...you get the idea.
If you're looking mainly for one for you to use, maybe provide some details on what you're hoping to develop in/on/for.
If you're mainly looking for suggestions so that you can compare them (and get a nice one as a side benefit), again do some research, and see if you can find common areas between the offerings, and compare on those. Anything that's not common, treat as either a unique feature/functionality, or as a minus, or as something that might influence niches of developers.