Final Fantasy Tactics has several things that make it particularly great:

You can recruit almost any generic foe onto your side.

You can breed your own army of monsters and poach them for rare cool items.

You have a huge cast of unique characters to play with, so you can always find characters that suit your gameplay style.

The most flexible job system ever that puts even FFV and FFX-2 to shame. Best of all, you can mix and match primary and secondary skill sets to your hearts content. You can have a Knight that casts Black Magic or a White Mage that can fight like a Monk. Endlessly addictive.

A lot of subquests and Propositions to keep you busy.

A great plot, but almost too elaborate for its own good.

Moderately difficult to beat, but a lot of effort to master everything.

I hope it doesn't sound like hyperbole to say that it's the closest of the Final Fantasy series to being flawless.