I'd say either Balnazaar (if it's a PVP server) or Stormrager (if it's a PVE server).

Balnazaar has a lot of people that know what multiboxing is and you won't be bugged too much about it (alliance side, at least). The battlegroup is great and I personally know a lot of great people there that're both awesome pvpers and who know that multiboxing ain't botting :P

Stormrager is also a fairly neat server, hordeside. It has a few odd fellas I've bumped into, but my tribox is in a guild called "General Goods Vendors" full of people who've rerolled and who all know what multiboxing is. The battlegroup isn't quite as good as the Balnazaar one and PVE progression isn't quite what it is on Balnazaar either, but the people here are very friendly.