Impossible to complete
Frontier: when you get half through, the span before another goal is achieved is geomethrically increased.
Utopia: try to beat the Lucratians in the last world!
The Immortal: this one was impossible even to start.
Almost completed without cheats
, Bubble Bobble, Deflektor
, Ghostbuster 2, Horror Zombies From The Crypt, Ishar, Lotus Esprit Turbo Challenge
, Troddlers Dragon's Lair 2.
the one i can think not on amiga is Guru Logic Champ (GBA) a great additive game
there are many i would like to complete, but if so, now i will cheat for sure
probably the only games i have finished without cheats are Moonstone (quite easy) Perihelion and Dune.