Probably WebKit. I worked on Safari, and while I had to make changes to WebKit very infrequently, the quality of the code, tooling, and team were excellent. I

Probably WebKit. I worked on Safari, and while I had to make changes to WebKit very infrequently, the quality of the code, tooling, and team were excellent. I haven't seen that level of engineering excellence anywhere I've worked since -- it was consistent and pervasive. No one had to enforce that culture from the top because everyone bought into the vision.