On EL&U, there is a 123-vote comment, and I'm wondering if it's the highest or not. How can I find out?
There is no normal way how can you retreive most-voted comment, however you can compose a query
Of course if you want the top EL&U score the query genesis linked to won't help because it only shows comments Greater than 300. Which not every site has.
This query Top Comment score just finds the top.
Because the data gets refreshed periodically The top comment on 6/6/2011 was on How do you quote a passage that has used '[sic]' mistakenly? so the 123 vote may be the current top.
As an aside if you ever want to change which SE site a query is run against you can just click the image next to the Run Query button for the site. Note it only appears after you run the query.