BA roundtrip going south, Santiago, Mendoza & BA

Planning a trip next Jan & require advice.
Thinking - BA south to Moreno & other good places - the into Chile - Santiago & back to Mendoza & BA.
3 weeks total. What is the best travel mode from the very south of Chile to Santiago?
What are the MUST see places?
Thanks
C

Man, you even suck at touting. The same two line message in every advertisement? You can't even tailor your posts to fit the OP? Do you really figure you're helping your business here? Idiot.