Well I'll hazard a guess using this sites Weekly Login Information http://lux-hdro.de/hdro-live.php A German Kinsite that compiles information from several websites such as Steam etc. and puts them together into very good graph content, you'll need to convert the page to English or be able to read German but I've been following the server statistics since 2014 and it looks as if the top 5 US servers according to weekly login data will be

1. Brandywine (confirmed)
2. Landroval (top 5 according to graph plus only US based RP server)
3. Gladden (has held the top 3 spot since graph point 239 which equates to around Christmas 2015 and a top 5 position since graph point 234/235 in fact Gladden according to this site held the #1 spot of weekly logins from graph point 197 to 229 approximately June to November of 2014)
4. Crickhollow holding top 4 spot since same timepoint of 234-235 Late December 2014
5. Arkenstone which has been holding the fifth spot since Late December graph point 234-235 of 2014

See graph included below. Helpful to visit the site and enlarge your browser window size to around 300%.

http://lux-hdro.de/images/live/weekly-logins-us.png