My go-to choices when solo are usually Hacienda de San Angel, sometimes San Angel Inn, Space 220, and Monseur Paul's. I haven't done Le Cellier or Biergarden solo - they just seem too crowded - and the seating at group tables in Biergarden just never struck me as worth the chance of getting an anti-social group or a group into their own conversations and not including another party. I also often head over to the Epcot Resort area and hit some of those restaurants like Fountain, Il Mulino, Beaches, Trattoria Al Forno, and Ale & Compass...all are a fairly easy walk from the park and you can walk right back in after.