Short description of an accomplishment

Congratulations in advance for when you become the most active voter on Stack Exchange.

If you're worried about losing your title to someone who casts a lot of close/delete/election votes, you could call yourself the most active up/down voter. But that's probably just the wrong side of the thin line between "committed supporter" and "nerd". I'd settle for the imprecision if I were you.

What about "I have cast the most total votes across all Stack Exchange sites."?

I'd suggest:

"First person to cast 50,000 votes on the StackExchange network."

(Or "family of sites" or similar, if you don't like "network.")