[Nightblade]

Master of Orion (the 1st one) has a very decent AI, i enjoy a lot playing from time to time and it always gives me a good challenge and does not seem to cheat in any ways.
playable under Dosbox for modern computers without any problems.

I remember reading that the AI in the RTS "Battle Realms" was too strong in the unpatched game and they toned it down a lot in the patches. I have no idea how it was, having only played the standalone expansion "Winter of the Wolf" that include an already patched Battle Realms, i would really have liked to try.
Winter of the wolf had a challenging skirmish AI though.

An immense disapointment regarding the skirmish AI for me was with Red Alert 2, it was so bad that even at the highest difficulty, it was actually very difficult to not cripple or destroy it.