I'm a 'tweener' on M4's right now (in-between deciding on a favorite). I'm looking forward to revisiting the Reiner on SACD when it comes out. Others:
Gatti- Lots of personality, nicely recorded.
Szell- Warm and affecting, but tromps all over Mahler's tempo indications
Maazel- A little cool, but often illuminating. Battle is very good.
Bernstein 2- Wonderful, boy soprano only partially succesful in finale, but still pretty endearing
Horenstein- Great character, but slowish. Margaret Price makes a beautiful, creamy contribution in the finale.
Welser-Most- Serene, but builds up to enormous power in climaxes
Inbal- Delightful, incorporates all of Mahler's tempo markings
MTT- Ultra-refined, gorgeous recording. Not really earthy enough for me.
Tennstedt- Alternately quirky and lush. Decent recorded sound.
Britten- Crisp, witty, and affectionate. Tolerable live sound. (mono, I think)
Of all these, I'm probably happiest with Gatti and Inbal, who actually go to the trouble of trying to perform what Mahler wrote.
I wish I knew which M4 Doc liked... Then maybe I'd listen to it tonight or tomorrow...
I'll report back.