Of the two Portal games, this one is my favourite. The story feels more developed, the world feels more lived in, and the puzzles get way more creative. The voice acting is absolutely fantastic and the inclusion of a co-op story mode was a great addition (it is a whole secondary campaign y'all). The first game is a MUST play before playing Portal 2, but if you're like me you'll remember this one more fondly once you've beat both.