That's a popular question that gets asked on this forum a lot. Skim the forum for previous threads but it really comes down to finding an area that you like to play, can survive playing and can play it fast enough to make spamming it worthwhile.
Castle Bluerock, Bloodpaw Cave and The Mausoleum and are often top three. Running Town Hall Quests (can get 5 to 8 at a time) is also doable.