1. Morrowind - could steal a few items in Vivec and buy training to max all skills in under 2 hours, at which point nothing could hurt you.

2. Daggerfall - skill gains were quick and didn't involve much thought, hard to argue due to the fact that it was made in the 90's.

3. Oblivion - Level scaling made it harder but you could still break the game easily with spellcrafting and enchanting 100% chameleon.

4. Skyrim - Still easy to break but you have to try harder to do it, training is more expensive and you need to level to aquire some of the better enchants.

Ooook? If you want a harder game, turn up the difficulty and play naked.