Chris Lewis of Nortel recently posted a breakdown of CBL listings by ISP in news.admin.net-abuse.email. Here's the top ten of his listing:

375649 chinanet.cn.net
130245 cnc-noc.net
102931 telekom.gov.tr
80936 kornet.net
67721 tpnet.pl
51671 dtag.de
47246 rain.fr
33678 interbusiness.it
33500 hananet.net
28433 hinet.net

The article itself can be found here (Message-ID <dmnk30$6m5$2@zcars129.ca.nortel.com>), and is worth reading for the full list.

The resulting subthread suggested that US-based ISPs are so low in the listings because many of them have blocks on outgoing port 25 connections from cablemodem and home DSL lines.

I'm a bit surprised by the list; I had no idea China was so bad (note that cnc-noc.net is Chinese), or that Turkey would be in third place. dtag.de (Deutsche Telekom) doesn't surprise me at all, as I have a number of systems that have been slammed by a large and aggressive collection of open proxies in their t-dialin.net and t-ipconnect.de domains.