Monitoring system recommendation

Dear Nanog community We are currently planning to upgrade our monitoring system (Opsview) due to scalability issues and I was wondering what do you recommend for monitoring 5000 hosts and 35000 services. We would like to use a monitoring system that is compatible with the nagios plugin format, however we are not sure if systems like Icinga/Shinken/Op5 are the way to go. Is someone using systems like Op5 or Icinga2 for monitoring > 5000 hosts? Would you recommend commercial systems like Sevone, Zabbix, etc instead of open source ones? Your input is really appreciated it Thank you and have a great day Regards