I’m gonna go with Boulderdash Construction Kit. Have spent so much time building caves! But there are many that are equally great: Lode Runner, Bagitman, Airborne Ranger, Bruce Lee, Defender of the Crown, Dino Eggs, Ghettoblaster, Infiltrator, Impossible Mission, H.E.R.O., Hacker…
I could go on, so many memories :D

Gateway to Apshai