Jens, one of the posters at Ionarts (which is based in Washington DC), has posted a very extensive survey of Mahler recordings at the WETA blog, with his personal preferences.
You can find it here:
http://ionarts.blogspot.com/2009/12/mahler-survey.html Of his recommendations, I own only ten. But then I only have a total of 71 Mahler recordings overall--a fairly small inventory.