What are some of the best FPS Single Player experiences apart from the Half-Life series?

Looking for some older games or hidden gems that have great SINGLE player campaigns with either great gameplay, story, concepts, or everything!

Half-Life is so good but there must be some others that if not on the same level are at least excellent.

Recommendations for some great Mods (eg Quake's Arcane Dimensions) of single player games would also be acceptable!

Mny thks

EDIT: Lot's of mentions of Titanfall 2....