I'm going with 2 slams as the most prestigious. Modern tennis is all about the slams. Even without much else in a career, 2 slams alone might get you into the Hall of Fame.
I think number 5 is the most difficult. Without any points, a player wouldn't qualify for any ATP events, including the Challengers. He'd have to shlep for very limited points in Futures events until he got enough to qualify for Challengers. Given the limited number of Futures events, this would probably take all of January and February even if he was winning big. Certainly wouldn't get direct entry into the clay Masters (even the quails) or the French so, again, even winning whatever low-level tournaments he played, he's not getting many points. The second half of the year, once he earned enough points to get direct entry into the ATP events, would have to be all time great level to get enough points to qualify for the year end finals. Someone correct me if I'm wrong, but I don't believe anyone has ever done this. Of course, if he somehow pulled a Raducanu at the Australian life would be much easier!
As for "greatest," which to me is an amazing rare and significant achievement which has very high status in the sport. Sort of a combination of difficulty/rarity and prestige. I'm going for YE#1 in singles and doubles. Every player views YE#1 as a really big deal and understands that it is very, very rare to do that in both singles and doubles. Lots of respect for that achievement. *
Martina Navrátilová held the #1 position in both singles and doubles for over 200 weeks!