You don't select a network management product based on it being the "best SNMP server". You need to take into account your requirements - not only technical but also budget and ability to support whatever tool you use.
For the simplest fault management (up/down status) many people like the free open source tools like Nagios. They then can add open source performance management with free tools like Cacti or PRTG. Depending on your network, something like Cisco Network Assistant can be used.
A popular entry level tool with a few more functions is What's Up Gold, a commercial product.
Full featured management systems that include not only fault but also performance, configuration etc functions include Cisco Prime LMS, Cisco Prime Infrastructure, Solarwinds' products (NPM, NCM, NTA etc.). Those can run into many tens of thousands of US dollars (capital expense acquisition cost) for a full enterprise wide deployment in a large network.
