Line 27, 89, 67, 83 run through
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 B of the RER runs through Luxembourg, right next to the Luxembourg garden and close to the Panthéon.