I started with Phrenology so I'd have to recommend that. Good album, accessible too. I've probably listened to Undun the most, no stand out tracks perhaps but a quality non-bloated album. Most of their stuff needs a few listens to reel you in imo. Black Thought is more gifted than most but not blessed with a hookshot voice.
I listened to Blackstar yesterday, that's probably my favourite Mos thing. I like the solo albums I've got of his (Black on Both Sides and Ecstatic), but I tend to pick tracks out of them rather than doing the whole thing.
Yeah, Ice Cube really runs out of steam really quickly. Some truly cringe stuff on there - I don't know what's happened to his flow but he sounds like he's forgotten about half the words in each verse.