If you give all 3 parks equal importance, then Riviera is most convenient, because it offers gondola access to Epcot and Hollywood Studios, and the Grand Floridian currently only gives monorail access to Magic Kingdom, and might or might not have monorail access to Epcot by late October. The gondola is more convenient than the monorail to Epcot, anyway.
They are both very nice resorts. If you want a spa, the Grand Floridian has one. If you want a view of the Electric Water Pageant (which is currently running) or Magic Kingdom fireworks (which are not currently going on, but might by October), then Grand Floridian has an advantage. Grand Floridian also has easy access to several good monorail resort restaurants, assuming that those restaurants are reopened by late October (several are currently closed). Riviera has Topolino's Terrace, which is very good. Riviera's pool area also has lovely views of the surrounding areas and the gondolas.