you can find the previous icml best paper awards by looking at the previous conference websites. for example:

http://www-ai.ijs.si/SasoDzeroski/ICML99/award.html via http://www.machinelearning.org/icml.html

in machine learning, i think the significant conferences are NIPS (http://www.nips.cc/), ICML, UAI (e.g., http://event.cwi.nl/uai2010/), and AISTATS (http://www.aistats.org/).