Legend of the Red Dragon (LoRD), Solar Realms Elite and Barren Realms Elite, and Tradewars were the best.

When I want to learn a new programming language, I always try to recreate Tradewars in it as a language. I know Tradewars like the back of my hand, so it allows me to focus on the nuances of the language while I build it. Such a fun project. The only thing I never quite figured out were the economics mechanics (it technically works, but it's a bit more predictable than TW2002 has in practice) and the Big Bang algorithm (I came up with my own, it's fine, but it doesn't have quite the same feel to it).

Less often, I'll try to create SRE/BRE, which, again is very fun but hard to reverse engineer. Amit (creator) lost the source code years ago, but wrote up some notes here: http://www-cs-students.stanford.edu/~amitp/Articles/SRE-Desi...

Funny, I just googled SRE/BRE to find the notes, and my last comment about it on HN was one of the top Google results... It's truly a lost art!