OGS (online-go.com) is the predominant server for players in the Western hemisphere, and provides a modern browser based UI. Fox has a larger player base, but that player base is almost entirely in Asian timezones. Fox requires downloading a not so modern client whose UI is lacking English translation. I get the feeling that Fox has a lot of bot players in the lower ranks.

It can take a few minutes to get a match on OGS. When that happens, I take the opportunity to warm up with a few tsumego drills on goproblems.com

Probably Fox would be the server with the most players but at DDK basically any would work and I think getting exposure to different playing styles would be worthwhile.