Which type of Nmap scan is the most reliable, but also the most visible, and likely to be picked up by and IDS?

A SYN scan is stealthier than a Connect scan because it sends only the initial SYN packet and tears down the half-open connection before it completes, generating fewer logs on the target.

An ACK scan is designed to map firewall rulesets by distinguishing filtered from unfiltered ports, not to reliably determine whether a port is open.

RST scan is not a standard standalone Nmap scan type used for open port discovery in the same way as a Connect or SYN scan.

A Connect scan uses the operating system's native connect() system call to complete the full TCP three-way handshake (SYN, SYN-ACK, ACK) for each target port, producing a definitive result for open or closed ports. Because the connection is fully established, it is logged by the target system's services and trivially detected by any IDS monitoring for completed connections. This combination of high reliability and high visibility distinguishes it from stealthier scan types that leave the handshake incomplete.

A FIN scan exploits how certain OS implementations respond to unexpected FIN packets, making it a stealth technique that is less visible to IDS than a Connect scan.