I've been gaming since NES so it may have been Dragon Warrior IV or something. The one that I remember devoting myself to for a while was Romance of the Three Kingdoms X on PS2.

It had my favorite battle system, engrossing RPG elements, a very interesting secondary larger-scale battle system known as Campaign Mode, and more.

Also, fans of the series may have a different take but I think ROTK X is the obvious zenith for the series. While other games before it were also great, the series fell apart by ROTK XI and XII and it just sucks now.