Field goal percentage, adjusted to account for the additional point scored off of 3-pointers. It seems that many people prefer AFG%, rather than raw FG%, as a measure. Personally, I prefer FG% because that tells you how effective a team is at making shots. That, to me, is the purest stat to measure how good a team is at offense. It doesn't take into account foul shots that can either be (a) given to us purposely at the end of close games or (b) come from a blown foul call, or (c) come from bad Harden-like "offense".
ORtg is, IMO, an inferior measure of offense. It measures points scored per 100 possessions, and takes into account bad fouls calls, bad offense, and fouls at the end of games that drive the score up. It also extrapolates out, similar to per36 numbers, and therefore is more of an estimate than a true measure.
In our case our ORtg is so high because we take so many 3-pointers. That doesn't necessarily mean we're a good offensive team - just that we play the odds on the 3-point shot. But when it comes to "putting the ball in the basket" we're average. We just make up for having an average offense by getting 50% more points (3 points vs 2 points) on 25% of our shots.
If you look at the standings for ORtg and compare to FG% I think you'll see that FG% is a better measure. POR, MIN, and TOR don't belong as "top offensive teams", and MIA/SAN are way too low.