If you're generating tracking URLs for your own site and you use Google Analytics, then I would recommend UTM parameters. They give you the most flexibility and are properly supported in Google Analytics.
If you're linking to other sites and you want to let them know you sent them, then using a ref parameter might be more appropriate. The reason for this is that not every site uses Google Analytics (and so UTM parameters might not be supported), and more importantly: Google Analytics won't like if you don't supply all required UTM parameters. (e.g. only utm_source)
In the past I used ?utm_source=betalist when linking to startups on #betalist, but we frequently received questions about this as it was generating error messages in people's Google Analytics dashboards. Switching to ?ref=betalist seems to have solved this.
The ref parameter will still show up in Google Analytics, but at a difference place. I haven't done much research into which one is most visible (in case that's your goal).
For what it's worth, I don't think ref is a standardized approach but it seems to becoming the convention. (some sites that use it include BetaList, Product Hunt, WIP, Places to Work, etc.
Thanks, the usecase for only using utm_source is very insightful
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