60s Series. It's just that it's too retro ish for me to watch (I can read older comics easily, they're not that cheesy). At second would come Spectacular Spider-Man, my 12 year old self could never watch it due to the art style back in the day, and it is a huge factor for me that my 12 year old self couldn't watch it. Third would be both of 80s Spider-Man series, they were like... Bleh. Never saw the 2003 series or Unlimited, so can't comment on them. Ultimate has evolved enough for me to like it. And I love the 90s Series, my 1-8 year old selves loved it, my 13-15 year old selves love it, and my current 18 year old self would agree on it.
For those wondering I actually have ADHD.