Final Fantasy Tactics with the latest collaborative patch at
http://ffhacktics.com/1.3/ is very rewarding and challenging, since you can't just level grind and crush everyone because all (or most?) of the encountered enemies level up similarly (bosses and unique generics have higher levels), so you have to rely much more on strategy to win, instead of mindless swarming like what would have worked for the original.
The patch involves much more than level-scaling the encounters, though.
The game mechanics have been completely reworked by total addicts who have invested countless hours into the game, removing some of the more exploitable and unbalanced aspects, and raising the importance of less-used skills, to balance the skillset out more and encourage (or coerce) the player to be more inventive with their strategy, since you won't have a single party/skillset that will work for every fight.
Also, the random encounters have been reworked too, where there are pre-made enemy parties for each of them, with various themes.
This is only for seasoned RPG players that can take the time to know game mechanics inside and out, but it's a blast, especially on the PSP where you aren't anchored to a TV and console.