The hardest turn-based RPG to me was probably one of the early Dragon Warrior games.

Oh, and what is with all of the comments about Xenosaga? I found Episode 1 to be fine in terms of difficulty.

Saga Frontier 2. That damn game was nigh impossible. The final boss had some ridiculous insta-kill / Insta-stone skills. Before the final boss, there was this one psuedo tactical battle mini game, this particular battle wasnt too hard, but once you defeated all of the regular units, some ridiculously powerful, impossible to kill units began to mobilize. Since the forces you had were static; it was essentially impossible to just level up and beat it.

I'd say any Shin Megami Tensei game in which Hama is an instant kill spell. Oh, and when it is, it's usually coupled with the wonderful concept of "main character dies = game over". Some parts of the Amala Labyrinth in Nocturne (Maniacs version) made me want to throw my controller at the TV.

Vagrant Story is ridiculous. Pretty much the only way to beat the hardest boss in the game is to be equipped with a really powerful two-handed sword, attack as much as you can, and then just before it uses its big ultimate attack switch to a really powerful shield.

Nah. The best way of surviving the final boss (which, I'm assuming, is the one you mean by the hardest one) - and really every boss in the game - is to understand the RISK system and your weapon's affinity. If your RISK reaches a high number you receive one hell of a blast to your HP when the final boss uses his uber-attack. If you keep your RISK low, you're only going to lose about half your HP. And keep in mind that enemies/bosses have multiple affinity stats (monster type, element, type of damage), and they are opposed. A Lich isn't likely to fall easily to a dark-affinity weapon, for example. The fact that you can forge new weapons in the game, coupled with one of the best soundtracks of all time and a great storyline makes VS the top game on my list.

the hardest ones i have played has to be Wasteland and Bard's Tale 2.
wasteland has to be one of the best turn based RPG's i have ever played. but very hard to get started. You would get ripped and beaten into a pulp the fist 5 exp levels if your not careful.

bards tale 2 was just a pain in the ass. every dungeon had fall through floors that a level 30 dragon\creature would spawn on. or if you stepped on a the wrong tile it would teleport  you to the entrance of the dungeon. or tiles that would spin you around and shift you in a different direction. i dont know it's strange those games were exciting because you never knew if you you were going to get your butt kicked by a random mob even if you were high level. SO ANti Oblivion or MMORPG's.

As a few people pointed out already...

7th Saga: You're playing alone most of the time. I remember the first fight against this Pison (I think) guy and lost after taking 2 hits. And I grinded. Got even tougher later on. I think I stopped playing after 6, 7 hours.

Secret of the Stars: There's about 16 characters to play with and still 11 of them are useless until very late in the game. You can switch parties anytime, but you never really need to. It's bad enough you get low exp. and grind for hours, the bosses still kick your ass with spells hurting the entire party. Mage users attack for 10, 15 damage tops in the end and their spells don't help them (100 to 200 damage tops). When you're about to finish the game, you need to grind another group for 10, 15 hours to get through a boss or two because you can't use your main party. This game is frustrating and not rewarding, unless you got 40 hours of your life to lose where you'll spend 30 of them grinding.

Final Fantasy I on the NES: I'm surprised no one mentioned this when they were talking about the FF series. I thought it was really hard. If all your characters attacked the same enemy and it died before the enemy party was killed and all of your characters used their turn, the system would not redirect the attack on another enemy. Instead, you'd waste a turn and got the grand prize of taking a beating from the other enemies. You could never know which equipment was suited for which character when you were buying, much less with spells (but I'm not 100 % sure about the last one though). System was slow, leveling up took forever too.

Phantasy Star I and II: Enemies hit way too hard even if you were overleveled... Pretty much the same as the others above.

I'd say any Shin Megami Tensei game in which Hama is an instant kill spell. Oh, and when it is, it's usually coupled with the wonderful concept of "main character dies = game over". Some parts of the Amala Labyrinth in Nocturne (Maniacs version) made me want to throw my controller at the TV.

You realize that in most SMT games where Hama is instant death, the main character is automatically immune to it, right? Like, I think the Persona* series and SMT3 are the only ones where Hama is instant death and can hurt the main character.

*Even then, P3 is the only one where the main character dies = Game Over

No, it's the same way in Nocturne. If skinny asian kid gets hit with hama it's game over.

Yeah every monster and their dog's brood has a death spell or two in megaten.

By the way, I think FF4 hardtype is impossible. Every attack Zeromus has is BIG BANG or NUKE or DESTROY or HARRY TRUMAN or something. Seems like he goes through the moves like tissues, too.

No, it's the same way in Nocturne. If skinny asian kid gets hit with hama it's game over.

Just FYI SMT3's full title is Shin Megami Tensei 3: Nocturne.