Line 4 runs north-south through the 6th, connecting with the 18th, 9th, 2nd, 1st, and 14th. If you're arriving from central France or the Loire valley you'll probably connect at Gare Montparnasse.
Line 27, 89, 67, 83 run through
Line B of the RER runs through Luxembourg, right next to the Luxembourg garden and close to the Panthéon.