[MichalSternik]

I would start with Philebus. It is considered a pretty late dialogue, so problems with theory of ideals are considered (and musical metaphors play a prominent role). Also, its topic is worthy; Socrates & Protarchus discuss a role that pleasure and reason play in furhishing a good life.

[MistahKoala]

Thanks for the recommendation - I'll have a look for it.