I will explain using an example of what I mean. Suppose I now start a new question on Stack Overflow and start typing in the tag editor textbox. I type "asp" and this is what I see:
Classic ASP has its place of honor, but these days it's rarely used — 130,000 is way bigger than 4500. I am subscribed to the asp-classic tag and see people putting it instead of asp.net almost daily.
Having the auto complete list sorted by the tag usage will help to prevent such newbie errors plus looks more correct to me. Might need some tweaking, e.g. don't put synonyms first but I think the general idea is clear.