I've been giving this some thought and I think I've arrived at an answer. The most annoying spam, in my opinion, is empty spam. This is an email where there is no subject or body - it's literally a blank email.
It's annoying enough when some sub-human mouth breather thinks it's okay to annoy millions of people to make money. In my opinion it's even more frustrating when it's pointless. The spammer can't even make money here. All they are doing is annoying a lot of people.
Nigerian scam emails are my person favourite because I can reply to them and string the scammers along for a while. Adverts for tablets are pretty entertaining as well since they tend to be written in diabolical English. Yes of course I'll take medical advice from someone for whom spelling a three-letter word is a herculean challenge.