Hi as the author of #SNMP, I try my best to be unbiased here :)
I have a blog post here which is a simple evaluation report.
http://www.lextm.com/index.php/2007/12/product-review-snmp-libraries-for-net-evaluation-report/
In my opinion, PowerSNMP is the leading one which has both complete feature set and simple/natural API. There are many commercial and open source products you can evaluate yourself to see which one meets your special needs.
Which one is the best? This bases on which subset of SNMP features you need and how big your budget is. :)