Rundgren has a gigantic discography; in addition to his own solo output there's his work with Utopia, plus the several dozen albums he's produced. A Wizard, a True Star is probably the most highly regarded of his prog albums, although I'm still unclear as to whether it's my favourite. The most highly regarded of his albums overall is Something/Anything?, which is unusual because it's also the longest item in his discography at over eighty-six minutes in length. For the most part, Rundgren has never gone in for short albums; the only item I have in his discography under fifty minutes is his Beatles parody, Utopia's Deface the Music, which is thirty-two minutes long, and which I would strongly recommend. Other albums I would recommend checking out are Todd Rundgren's Utopia, Todd, and Initiation; Healing also has some interesting moments and if you're up for his standard pop, Hermit of Mink Hollow is essential listening as well. I'm also told by reliable sources that Utopia's Another Live and Ra contain essential prog, but I've yet to listen to them personally because I've been too obsessed with Deface the Music. His last two solo efforts have also gotten excellent reviews, and I can vouch that Liars is quite well executed pop music. Arena is reportedly more of a throwback to the hard rock sound of Utopia's latter days, but I haven't listened to it yet.
So what to start with? It's up to you. Initiation contains a really interesting exploration of synthesizer music on the second side which I find to be enthralling, but some people regard as self-indulgent crap. The same goes for the army-of-keyboards approach on Utopia's début, where three of four tracks exceeed ten minutes and one of them exceeds thirty. Of his more song-oriented albums, the songs might be most consistent on A Wizard or Something/Anything? so those are definitely good entry places; it's hard to go wrong with tracks like "Zen Archer," "Just One Victory," "International Feel," "Hello It's Me," "Breathless" and "I Saw the Light." Deface the Music might also be a good starting point, if only for the awe it evokes at how effectively Rundgren and company manage to make excellent music in the style of so many periods of the Beatles' music.