If you haven't played it already, Final Fantasy Tactics: Advance is one of the best games outside of the main series IMO.
Final Fantasy Tactics is one of my favourite games of all time, but I strongly disliked the GBA sequel. The brilliant Game of Thrones tale of political intrigue is replaced by a bunch of silly crap for kids. The job system is butchered and I'm not even going to get into the judge system. Oh, you so don't want me to get into that.
I'd say Tactics Ogre: Knights of Lodis is a much better SRPG on the GBA.