There are a few I like but for the constant mainstays: Leviathan, Odin, Bahamut, and Alexander are easily my favorites. These were my favorite appearances for them.

Leviathan (IV, VII, VIII)

Odin (IV-IX)

Bahamut (IV, VI, VII, IX)

Alexander (VI, VII, and IX)

For not so traditional or not so consistent, I usually like Golem and Titan, Fenrir, Hades, Syldra, Mist Dragon, and most of the XII espers.

As I currently play through FFIV:TAY, I'm reminded how much I love being able to obtain normal monsters as summons and kind of wish FF would bring that back.