Well, I'll start with one strength for each game, but if the thread is popular enough there is certainly more for me to bring up.

Final Fantasy Tactics: Plot. While it might seem odd coming from myself, I actually like a lot of the storyline of FFT. The War of the Lions is a truly epic confrontation, and your little band fights through an incredibly deep and moving story of betrayal, murder, and manipulation. It had some aspects that truly annoyed me, but overall, an incredibly good story, with quality, detail, and length that I would like to see in the next FFTactics game.

FFTA: Combat. Excluding the laws, FFTA's combat system was brilliant. It brought some much needed balance to the job system, and encouraged versatility and growth. Combat was swift, brutal, and fun, until you got to the heavy law sections, which were far too restrictive. FFTA's combat in general (minus laws) beats out TA2's, IMO, because TA2 unbalances the job system again by forcing magic users to start with zero MP, and workarounds for that were too restrictive.

FFTA2: The world. Being placed in the newly envisioned FFXII Ivalice gave this game some nice set up, and let the writers just have fun with the world. They took every opportunity to make the world as incredible as they could. From their villains, to their comic relief characters, to their bit-part players, ever NPC was written well and none of them seemed out of place. The game balanced light-hearted fun with dark emotional points, and generally did everything it could to make Ivalice a place that I just love to explore.