Can vary pretty wildly. I'm very average myself, around 1.1 K/D but that is usually a mix of doing well (1.5-2.5) in some matches and badly in others (0.5-0.8). Sometimes I suck because I'm trying to do a bounty (just running around punching everyone), sometimes just because I suck during that particular game. Highest I've seen on a player I've run into in a game (not looking at a random profile online) is around 3. But it's hard to look at people's stats during a game, usually there is not that much time between matchmaking. I think it also can vary based on whether the person runs with teams, or if they go solo in pick-up/random games. I wouldn't be surprised if a good player might get, say, 1.5-2.0 in random games, but jump up to a 2.5-3.0 if they are with similarly skilled teammates in a fireteam.