I have used Elliot's commentary. It's aimed towards a less experienced Greek reader and therefore has more helpful comments on parsing grammar, unusual constructions etc than is typical. It also has a vocabulary in the back which is both useful and frustrating, as it doesn't list vocab considered too basic and when a word is listed it sometimes requires some digging around to find it.
The commentary is sometimes annoyingly brief, however, and I ended up supplementing it with Page's commentary, which was often more helpful on particularly difficult passages. Some of this is an individual thing though, what things one has trouble with and what not.
The Cambridge Green and Yellow (I don't remember the editor off-hand) is good, I think, although I didn"t have a copy when I worked through Medea the first time. It's quite thorough and is again aimed at an intermediate-level Greek reader. If you only want to get one commentary this is probably the one I would choose.
IPHIGENIE: Kann uns zum Vaterland die Fremde werden?
ARKAS: Und dir ist fremd das Vaterland geworden.
IPHIGENIE: Das ist's, warum mein blutend Herz nicht heilt.
(Goethe, Iphigenie auf Tauris)