This question does not ask for a solution but rather is a poll for opinions. I will split the points by the quality and comprehensiveness of answers. I hope it's ok and you will be willing to share.
We're planning a new medium data center (about 5000 computing nodes). We're looking for two main objectives: price/computational performance and price/easiness of maintenance. Based on your experience, what server hardware have performed best for you in such goals? Specifically, how did your servers (pizza, multi-node in pizza chassis or blades) perform out-of-box, i.e. what percent of them proved to be faulty and required hardware replacement (disks, RAM, network cards, motherboards), and then, how did these servers withstand intensive computative and i/o environment? The rate of failures in two of these situations is most interesting. The approximate setup of a typical server would be a 8-16 cores, 48-96Gb RAM, IPMI and 2 LAN ports.
We're talking x86 (Intel, AMD)-based servers, and the brands would naturally be:
* Anything else?