Having tried a number of RBLs you will see that some are going out of existence, some are either too aggressive or just do not cover all of the areas that give you the protection you need.  Here is a list that makes a good combination of protection without getting too crazy.  Be sure to try one at a time so you can determine if one is not right for you.  Use at your own risk…you could lose email.

zen.spamhaus.org
This list contains three separate lists.
“The SBL is a realtime database of IP addresses of verified spam sources and spam operations (including spammers, spam gangs and spam support services), maintained by the Spamhaus Project team and supplied as a free service to help email administrators better manage incoming email streams.
The Spamhaus Exploits Block List (XBL) is a realtime database of IP addresses of hijacked PCs infected by illegal 3rd party exploits, including open proxies (HTTP, socks, AnalogX, wingate, etc), worms/viruses with built-in spam engines, and other types of trojan-horse exploits.
The Spamhaus PBL is a DNSBL database of end-user IP address ranges which should not be delivering unauthenticated SMTP email to any Internet mail server except those provided for specifically by an ISP for that customer’s use. The PBL helps networks enforce their Acceptable Use Policy for dynamic and non-MTA customer IP ranges.”

bl.spamcop.net

SpamCop Block List
This list contains IP Addresses which have sent Spam as reported by users themselves.  This provides the advantage of a list that is finely tuned and very up to date as users respond to add IPs to this list.  However, it is an aggressive list as they state.

“The SCBL is aggressive and often errs on the side of blocking mail.”

The other disadvantage is that any user can add an IP to the list thus creating a serious problem for an organization whether it is justified or not.  Your enemies or competitors could use this list against you.

cbl.abuseat.org
The CBL takes its source data from very large spamtraps/mail infrastructures, and only lists IPs exhibiting characteristics which are specific to open proxies of various sorts (HTTP, socks, AnalogX, wingate etc) and dedicated Spam BOTs which have been abused to send spam, worms/viruses that do their own direct mail transmission, or some types of trojan-horse or “stealth” spamware, dictionary mail harvesters etc.

dnsbl-1.uceprotect.net
This is a unique list in that it will provide results of spammers listed in the last 7 days.