I personally use a three tier approach that catches 99% of the attempts:
Registration Attempt ->
Stop Forum Spam Check -> Proxy RBL Check
Post Attempt ->
Akismet Check
For RBL I'm a little more aggressive on the IPCONFIG checks that the default setting:
- dnsbl.ahbl.org
- list.dsbl.org
- sbl-xbl.spamhaus.org
- cbl.abuseat.org
- bl.spamcop.net
- dnsbl-1.uceprotect.net
- dnsbl-2.uceprotect.net
- dnsbl-3.uceprotect.net
- zen.spamhaus.org