These would be my favorite live Mahler performances (in no particular order):
Barenboim / Staatskapelle Berlin - M5 (Philharmonie, Berlin)
Barenboim / Staatskapelle Berlin / Fritz, DeYoung - DLvdE (Philharmonie, Berlin)
Barenboim / Staatskapelle Berlin - M1 (Philharmonie, Berlin)
Jansons / RCO - M6 (Großes Festspielhaus, Salzburg)
Jansons / RCO - M1 (Philharmonie, Berlin)
Abbado / Lucerne Festival Orchestra / Larsson - M3 (Royal Albert Hall, London)
Gilbert / DSO Berlin - M6 (Philharmonie, Berlin)
DeWaart / DSO Berlin - M7 (Philharmonie, Berlin)
Rattle / BPO - M9 (Philharmonie, Berlin)
Rattle / BPO / Koczena, Isokoski - M2 (Philharmonie, Berlin)
Rattle / BPO - M10 (Cooke) (Philharmonie, Berlin)
Worst (i.e. most boring ever):
Mehta / BPO - M7 (Philharmonie, Berlin)
Also disappointing:
Eschenbach / Philadelphia Orchestra - M1 (Stadthalle, Braunschweig)
Maazel / NYPO - M5 (Stadthalle, Braunschweig)