Here is a list of the Top 5 Port Scanners.
1. Nmap
A versatile port scanner , has got lot of scanning options. Can perform a variety of scan (syn , fin ,ack etc.)
Not very fast but is very accurate
also can detect host OS
2. Superscan -
Fastest port scanner i’v seen
Can scan tcp/udp ports . also has many network utilities like ping ,tracerouts, whois etc.
3 Angry Ip Scanner
Angry IP Scanner can perform basic host discovery and port scans on Windows. Its binary file size is very small compared to other scanners and other pieces of information about the target hosts can be extended with plugins
4.Unicorn Scan
Unicornscan is an attempt at a User-land Distributed TCP/IP stack for information gathering and correlation. It is intended to provide a researcher a superior interface for introducing a stimulus into and measuring a response from a TCP/IP enabled device or network. Some of its features include asynchronous stateless TCP scanning with all variations of TCP flags, asynchronous stateless TCP banner grabbing, and active/passive remote OS, application, and component identification by analyzing responses. Like Scanrand, it isn’t for the faint of heart.
5 Scanrand
An unusually fast stateless network service and topology discovery system
Scanrand is a stateless host-discovery and port-scanner similar in design to Unicornscan. It trades off reliability for amazingly fast speeds and uses cryptographic techniques to prevent attackers from manipulating scan results.