Not really a "Simple Question / Answer" but I just did some calculations in Numbers (Excel for Macs) and thought I'd post some data in case anyone else was curious was as curious as me.
I copied a pokemon type chart into an excel sheet, and used that to generate weakness/resistances for any pokemon (single or dual type). Then I took the usage data from
here and typed out the top 50 pokemon along with their proportionate usage and typing. Finally, I went through each attacking type and ran down the list of VGC pokemon, multiplied their weakness/resistance multiplier times their usage percent and divided by total usage to get a "somewhat" accurate idea of which attacking types are most effective in the metagame.
The top result is actually Ice, which hits for an average of 137.05% damage. The top two pokemon (Chandelure and Jellicent) both resist it, but it hits super effective against numbers 3, 5, 6, 8, 10, 11, and 13. The rest are below:
Ice 137.05%
Flying 121.77%
Rock 115.17%
Dragon 109.60%
Water 109.47%
Ghost 108.34%
Fighting 103.43%
Dark 102.54%
Psychic 102.42%
Ground 102.24%
Fire 99.15%
Electric 96.23%
Grass 94.90%
Steel 93.63%
Bug 91.51%
Poison 85.58%
Normal 77.98%
Excel sheet is available
here if you want it. I haven't tested to make sure it works, since I worked in Numbers and saved a copy as an xls file for the benefit of the PC users. Let me know if you want the original numbers file.
EDIT: Keep in mind this doesn't account for abilities like Levitate, Flash Fire, Water Absorb, etc, so it's not 100%.