Gang hideouts... specifically Twin Rocks or Pike's Basin. Twin Rocks is best with 1-3 people, as it is small and can be completed quickly over and over again once you get good at it. If you have 4 or more people, however, many more enemies will spawn there, so it's usually best to keep it to 1-3 players. Pike's Basin is huge, so doing it by yourself or with only a friend or two can take a long time. But if you have a larger group of people playing it and you have a good strategy, it can be great. These two hideouts are the best for quick leveling, but if you get sick of them, Tesoro Azul and Fort Mercer are pretty good too. You can also get some decent XP for doing co-op missions or Undead Overrun, if you have it, but gang hideouts are by far the best. Also, there are triple and quadruple XP events from time to time, so check the official Rockstar site for RDR from time to time so you don't miss them. Hope this helps!