Space 1889. Steampunk done right and very accurate to history if you strip away the Verne and Wells. The most fun and enjoyable setting for a campaign I've ever played. The original rules were very clunky, but the new edition is more streamlined. Possibly has the best sourcebooks I've ever read.

Traveller. Best hard sci-fi system. Nothing else comes close.

Cyberpunk/Shadowrun. Very cool settings with lots of atmosphere. I haven't played the recent editions but you played the old editions despite the rules because of the setting.

Barbarians of Lemuria. Rules light Conan style adventures. My number one introductory RPG as well a fun and relaxed environment for grognards.