Nagios vs Zabbix - Which one is best?

Both are nice monitoring tools for Linux OS, monitor ports, sites, processes etc and generate caution via email/ SMS etc.

I haven't work much on Zabbix but features wise, it has nice Web interface, graph presentations is an extra benifit.

Where as in Nagios, you can define custom settings to suits yourself, what you are looking for. e.g execute a bash script and if it doen't generates certain output, can generate an alert (send a mail/ SMS)

I personally find Nagios is more flexible (define custom checks). 

Nagios is good option for monitoing. Can be intreted with multiple applications.

For me Nagios, since I built one from scratch to implementation.

I have used nagios in realtime so feel nagios is good tool to monitor network devices like windows server, linux server, switch, router and firewall

i recommand centreon engine and broker , with a lot of wxidgets and ad-ons to generat reporting

I think Both are good. Some points about those two tools:

Zabbix has a web configuration, while Nagios uses text files. While web configuration seems nice, it is a pain to automate like using Chef or Puppet.

Zabbix has graphs and stats collection via SNMP or custom agent. Nagios don't. Even so, you must think if you are really looking for graphing support or just a robust monitoring system to tell you if something goes down.

Zabbix is resource hungry. It uses a DB to store all the stats and configuration and this can grow exponentially depending on the number of hosts you are monitoring. Nagios works very fast and normally will not require the ammount of resources that Zabbix does. You can tune zabbix, of course, but it's more work to do.