Which is best NEtwork Simulator for CCNA,CCNP,CCIE?

Well, a simulator only simulates or 'tries to' simulates a certain feature or set of features as close to the actual product as it can. But, still as it is not 'emulating' (running/try to run the same hardware, same fixes, same code, same OS), so that eliminates the keyword 'Best' in your question. 

As of my knowledge, one can never say that 'this' network simulator is best or is better than others. It depends on the scenario you want to put it through. 

Having said that, when it comes to CCNA/CCNP you can have two ways to go 

1. Cisco's Packet Tracer (that simulates )
2. GNS3 (that actually runs the Cisco IoS)

But, there's a catch to it. If you have Cisco's Packet Tracer, its pretty straight forward to make a topology on it and configure it in either GUI or CLI mode. You can also try to simulate packets and other set of features in real-time environment. 

Packet Tracer works well for CCNA and I do not know any particular lab of CCNA curriculum that you cannot perform on it. 

When it comes to GNS3, its a little tricky. first off, you have to have Cisco's IoS that you want to hypervise in GNS3 (we will not be discussing 'how to get IoS' here). But, assuming that you have everything ready, GNS3 kind of gives you a great feel to actual network. You can only drag/drop routers and network devices on its interface and from there onwards, its pure CLI (something you need to master if you are going for CCNA or higher exams).

Also, as far as I know, everyone at CCNP or higher level prefers using GNS3 if they do not have actual equipment.

When it comes to CCIE, its a completely different story. A few years back there were no simulators or emulators for advanced CCIE labs and you either had to buy or rent the equipment.

Currently, you can 'try to' emulate a lot of that.

 You could look into :

1. Cisco's VIRL (which covers Cisco IoS and Cat OS) - a bit pricey
2. Titanium that 'tries to' run NX-OS features

I think from my experience the best is GNS 3

As I know GNS3 is the best in this filed because it is does not simulating, it is emulating which means that it create a virtual router on your computer hardware to run the a real copy of the IOS.

GNS3 is one of the best if you are intermediate. For beginners packet tracer will work.

You can use Netsim network simulator  but it is very expensive simulator out there! or better you use GNS3 because it uses the real cisco ISO.

GNS3 and Packet Tracer

some features y can not apply on Packet Tracer like named eigrp, qos path control.

GNS3 more reliable for all features.

Packet Tracer is enough for CCNA

& GNS3 with IOU for CCNP & CCIE.

1. Cisco's Packet Tracer (that simulates )
2. GNS3 (that actually runs the Cisco IoS)

I think for beginners Packet tracer is the best  simulator program and for advanced user physical layer 3 switch is very helpful to learn CCNA.

for CCNA & CCNP i would say GNS3 and packet tracer and for CCIE you can use GNS3 plus rack rental service from INE or cisco it self.

I have used all the simulators except UNETLAB. I have used Packet Tracer, Boson, GNS3 and VIRL for my CCNA RS till CCIE RS.

CCNA - Packet tracer has all the functionality one needs for CCNA and you can ace the exam with this simulator. It takes around 5 months for a non-IT person to learn it.

CCNP - Here you need GNS3, Packet Tracer and real switches. You have three exams. Route - you can practice 95% of all the commands here in GNS3 and ace the exam! Switch - you need real switches but some functionality around 50% can be done in packet tracer.

CCIE - This is a little difficult as you need routers and switches together. You can build a real lab or use VIRL. I personally recommend VIRL as it can do 99% of CCIE RS commands. I'm using this. It's very good. That's the advantage but the disadvantage is that its 200$ per year and requires a lot of hardware to run the images. You need minimum of 6 Core CPU and 16GB RAM in the virtual memory for around 10 routers which is good enough for INE's CCIE workbook lab. I hope this is useful for you. If you need any help contact me.

Thank you,

Arsalan