A.M. - pretty straightforward alt-country
Being There - alt-country mixed with alternative
Summerteeth - poppy rock with a hint of their old sound
Yankee Hotel Masterpiece - this is usually considered their masterpiece. It's basically the sound from Summerteeth combined with a good deal of experimentation.
A Ghost Is Born - there isn't anything remarkable about this album, to be honest, it's basically a typical Wilco album with a marginal amount of experimentation.
Sky Blue Sky - has a "dadrock" air to it. Nice guitar work though.
Wilco (The Album) - this sounds like a mashup of every Wilco album yet, minus A.M.
As a starting point, I'd personally recommend Summerteeth and Yankee Hotel Foxtrot. Those two along with Being There are my favorites.