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There are countless processors that you can choose for for this build. I'm going for something that's middle of the road" the Intel Core i5-3570S.
This is a quad-core processor which runs at 3.1GHz, and can be turbo boosted to 3.8GHz. It has 6MB or L3 cache and a maximum TDP of 65W, making it ideal for use in the all-in-one PC.
Image source: Intel.