Rayder said:
It doesn't get much more difficult than Etrian Odyssey.
Dark Spire is a difficult old-school RPG.
Actually, I think SMT

S is way harder than EO and TDS.
in EO, you could grind your way through the game.
also, in EO I there were these kick-ass ways to play the game, the best of which is found in a topic discussing EO II here, at gbatemp.
In TDS...well, it is hard, but you could just save and reload etc, and bosses were ridicoulously easy.
SMT

S, however has:
An Experience limit: For some reason, my Hero gets 30 EXP from a fight that would give my team mates 300 exp.
this makes grinding almost impossible.
EDIT 3: FF4 is awesome, too.
And you should try out SMT:SJ. It's coming out on ten MArch, 2010, but it's playable in japanese, too, if you can't stand the wait.
Also, TWEWY. not particularly hard, but it's really awesome.
also, the enemies love status effects way more then EO's enemies.
you also have to worry about weaknesses a lot more then in EO I/II, and Status effects were almost no-existant in TDS.
Getting on-topic, though: EO I/II, TDS, and SMT

S are excellent games.
Love the former three and currently getting my ass handed to me in the latter.
If you've already played those games, however (and you appearantly have),
then I can't help you, as I can't think of any really difficult DS games other then these four.
EDIT: also, you're axtremely awesome if you liked these games.
no kidding dude, you've got more balls than 9/10 of all gbatempers here.
check the threads of EO I/II and TDS: people are bashing those games due to their "difficulty",
and call it shit because they can't handle the fact there is an enemy wandering around that isn't meant to be attacked,
but to be avoided.
EDIT 2: oops, just checked your backloggery, and it says you've already started SMT

S.
Have fun, then, and don't forget EO II and TDS!
EDIT 3: FF4 is hard,too.
just like SMT:SJ. it's coming out on 10 march, 2010, but it's really playable in Japanese if you can't stand the wait.
Also, try TWEWY. not particularly hard, but it's really awesome