There are many amazing SSI games, but i have a special place in my heart for some of the lesser known ones:
Six Gun Shootout -- i would make use of the character editor to go through and convert the game into an 'rpg' where my self-made character would go through every mission and 'level up' based on his kill count. I'd just take an existing character, change his name and stats, etc. iirc they even suggested doing this in the manual. I played through the missions so many times I can still picture some of the maps decades later. Few games captured that sense of tactical bliss like this game did, busting through a door, dropping to a knee, and letting loose with a sawed-off shotgun in western turn based goodness. I remember even playing this w/o a numberpad for a long time, meaning i had to memorize the positional relevance of the number keys to move. So good. Getting to replay classic western scenes in history and film as a kid? Priceless.
Roadwar 2000 / Roadware Europa -- going through the wastelands acquiring a small army of soldiers to man your convoy of vehicles. Nothing like finding a huge truck and filling it with gunners.
The Shard of Spring / Gemstone Warrior -- 'lesser' rpgs in some ways, but I still remember both vividly.
Galactic Gladiators, Eye of the Beholder, Gemstone Warrior, etc all deserve honorable mentions.
SSI, you've been missed.