Fastest way for me to rank up my characters is through the crucible. I've hit level 20 in a day or 2. Depends how much and how long you play tho. Plus those bounties for the crucible also help. Each kill is around 60 experience. So if you kill 10 people each match that's 600 plus however many you assists you get and however many zones you capture in control. You level up fast. I have a 28 lock and 31 hunter that hasn't even unlocked Venus yet. Lol