Nintendo has confirmed via the wonders of Wario Ware Gold how you correctly pronounce 'NES,' something which has been a source of contention among fans pretty much since 1983.
According to the Wario Ware Gold listing for the Nintendo Entertainment System, the Big N clearly states that it's pronounced 'Ness.'
To be honest, I didn't even realise people were arguing about how to say the bloody word. I always pronounced it 'Nez,' although apparently I was wrong all this time and can now say I've seen the light. Cheers, Ninty.
The NES has been back in the public conscious lately thanks to Nintendo restocking the NES Classic Edition, after its initial stock sold like hot cakes.
In fact, it's been doing so well that it trounced all other hardware in the US back in June.