I have been an ELO fan for years and years...
I would recommend Out of the Blue.
Originally released as a two record set (back in the days of vinyl. Mr. Blue Sky is from this album. In fact, the side that Mr Blue Sky is the last song on contains four linked tracks, with the entire side called Concerto for a Rainy Day.
The set also has some of the fantastic stuff that made ELO great. One track - Birmingham Blues - which Jeff Lynne dedicated to his favorite football squad has a violin riff that makes you think "I have hear that before"..it's a riff of the main melody from Gershwin's Rhapsody in Blue. And Lynne makes it work.
ELO rocks. Period.
PS one of the best sites for all things ELO is
http://www.rockaria.com/elomain.htmlPSS ...almost forgot another one to recommend, their 1981 concept album called Time...