This is actually one of my favorite genres: I'd definitely recommend the Final Fantasy Tactics games and Fire Emblem:
Since you obviously seem to have a DS, Final Fantasy Tactics Advanced and both the Fire Emblem games for the GBA were some of my favorites. The DS sequels (Final Fantasy Tactics Advanced 2 and Fire Emblem Shadow Dragon) were pretty good, though not quite as good.
In the Final Fantasy Tactics series, the PSP port (or for that matter, the iPhone port) of the original Final Fantasy Tactics is also worth checking out; though the game has "aged" a little bit. (You could also look for a PS1 copy of FFT, but I personally have a hard time playing these games on -non-handheld devices, I'm afraid)
I'm working my way though Final Fantasy Tactics again, and then I'm going to move on to Tactics Ogre (PSP), which I've heard is one of the best in the genre. (I'm doing FFT first because I'd rather play the weaker game before the stronger one)