I’m a big fan of Zabbix. After spending some time with Icinga, when we upgraded our hardware infrastructure I moved over to Zabbix and I felt much better with it. It has almost everything you need, you can plot graphs and read historical data out of the box. Most of this stuff cannot be done with the base Nagios/Icinga.
Zabbix just released their new major version 3, which includes a brand new web design, encryption support and much more. You can read all details at their website.