My proudest work with a large studio and publisher was building multiple levels for the PC game Darkspore. Darkspore was a project EA/Maxis had started after their previous title Spore. The idea was rather than create the "mile wide and a foot deep" gameplay of Spore, Maxis would use the Spore creature creator and combine it with in isometric action RPG game. The premise was that players would build creatures, pick up parts, and attach them to their creatures for stat bonuses.
I was primarily responsible for the Scaldron levels from inception to finish. However, my hand also helped with almost every level in the game. I worked very closely with my two fellow level designers and the art team to deliver the Scaldron levels as well as the other levels in a very tight schedule. The team for Darkspore was around 50 people, which was great because we could be very agile with our development approach.
Maxis was hands down the best development environment I have ever worked in. The tools and engine had very few technical problems and rapid iteration was the name of the game. My co-workers were also some of the most amazing and professional people I have ever had the privilege to work with.