I completed TMNT as a kid... on Commodore 64. That version is admittedly a little bit easier than the NES version (some mechanics were missing, and an entire level is gone, as I recall). Still, I have no idea why people complain about the second level (river), it's actually pretty fun. Compared to what's to come later in the game.
To me, the definitive "hard" game is Metroid Prime 2: Echoes on GameCube. Dark Souls just makes me say "eeeeeehhhh this is probably doable, I'll play this after I'm done with MP2E."
(When I first played MP2E, I only got through the second to last boss. Then my MadCatz memory card died. Played through the game again, with the fury of million suns. 99% complete. Because I missed one optional scan. ...One of these days I replay this bastard.)