Sophos has published their top 10 list for 2004 as follows:
http://news.bbc.co.uk/2/hi/technology/4105007.stm
TOP VIRUSES OF 2004 1) Netsky-P 2) Zafi-B 3) Sasser 4) Netsky-B 5) Netsky-D 6) Netsky-Z 7) MyDoom-A 8) Sober-I 9) Netsky-C 10) Bagle-AA