I use the Garmin Connect "Correction Enabled" data. Not necessarily because it's the most accurate, but because I simply want consistency, and it seems to be pretty consistent when comparing identical rides/routes (like commutes).

For instance, the elevation for my commute to work can be anywhere from 90' to 165' elevation gain. The Garmin barometer usually gives me about 150-155'. Garmin corrected data gives me about 125-130'. I just count it at 120'.

Just for fun, I picked a few landmarks on an 1,100' climb near my house, and got the elevation data from Google Earth. When I passed these spots, I looked down at the Garmin (Edge 500), and the elevation that it showed me was spot on...but that doesn't necessarily mean that the gain/loss will be perfectly accurate.