Elixirs (extra light) when I can.

The Elixir folks really do stand behind their guarantee that they'll last 3 times as long (without breaking or going bad) as other strings or they'll give you your money back. I had a couple of Elixir strings break fairly quickly a year or two ago. I'll never completely know why (though in retrospect I suspect it may have been a problem with my Sigma's plastic saddle), but when I called Elixir about it, they gladly replaced both strings and sent me two brand new sets to boot.

The Elixirs do last longer, and I like the way they feel.   But I also buy some high E strings and some B's in bulk, since they break most often. I also keep a few $2 and $3 sets of strings (Dean Markley, Martin, D'Addario, etc.) around as spares. I don't usually buy the Elixirs unless they're on sale.