If there are engineers you highly respect on your team, their code is a great place to start.
Otherwise, many top tech companies now open source software that they've written internally, oftentimes with their own websites. And there is also a growing trend for them to actually maintain the software they open source as opposed to just throwing it over the fence.
Think of companies that have a strong engineering brand, and then just search for what open source software they're released. Pick whatever seems most aligned with your interests.