which is the best open source or paid network monitoring tool ?
You could check the link below for commercial and paid options. It really depends on the amount of features you wish to do as well.
There is no clear winner in this category.
This is a wide open question.. There are tons of tools available, but it all depends on what you want. Cisco has their network management products, like NCM, Ciscoworks etc, which can do specific tasks. Have a look at the datasheets to find more about what they can support.. There are other non-cisco products which can be looked upon.. but deciding a network management system is a big task, comprising of price, feature support, licensing, no of devices to be supported etc..
Hope this helps