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

SYN (half-open) scan never completes the handshake and tears down the connection before it is logged, making it significantly stealthier than a Connect scan.

ACK scan is used to map firewall rulesets and identify stateful inspection rules, not to discover open ports via full connection.

RST scan is not a standard Nmap port scanning technique used for port discovery.

The Connect scan uses the OS-level connect() system call to complete the full TCP three-way handshake, guaranteeing accurate port state results without requiring raw socket privileges. Because the connection is fully established, it is recorded in the target's application and system logs and is trivially flagged by any IDS monitoring for completed TCP connections.

FIN scan sends only a FIN packet without initiating a handshake, making it stealthier and far less visible to IDS than a Connect scan.