That's kind of a tough one. While I love ELO, I've never been satisfied with the sound quality of their work. Out of the Blue was actually the very first album I ever bought back when I was around 13. It's my favorite ELO album, but it's better when it's not critiqued with a hyper-critical ear. I like it better in my car, for example, than with my cans.
That said, I think Out of the Blue and Discovery are the two "best-sounding" ELO efforts. Time and Secret Messages both sound pretty crappy to me, even though I like Time a lot.
You should also give ZOOM a try if you like Jeff Lynne's work at all. It's a departure from old ELO, but some of the songs on that album are really great. From a sound quality standpoint, however, it's certainly nothing to get excited about.
I would love to see some kind of remastering project of ELO's catalog, including some high resolution releases, or SACD multichannel versions. Something that adds more clarity and depth would be very welcome in my household.