From memory (which is fading fast lol), the games below I've definitely completed without any cheats:
Beneath A Steel Sky
Indiana Jones and the Fate of Atlantis
Simon the Sorcerer
Lemmings 2: The Tribes
Lotus (all three)
Another World (what an ecstatic feeling I got on completion)
Outrun (Poor port I thought though and looooong loading times)
Shadow of the Beast
All the Monkey Island games
Rodland (didn't find that difficult)
I couldn't tell you how long in hours it had taken me to complete any of the above. Most of them I had taken a break with and returned to a few days/weeks later.
Probably a few more I had completed but possibly with a cheat of some form. Examples: Project X (one tough cookie!), Chaos Engine, Turrican, SWIV, to name a few.