Career omisson - whose is the most painful?
No-one in tennis has ever won everything (not yet anyway). Every top player has something they haven't won. It would be interesting to see how people see the most glaring omission from former number 1's achievement and rank the most glaring omission.
Laver - No WCT Finals or Masters
Federer - No Olympics singles gold (yet)
Nadal - no year end final championship
Borg - no US Open
Agassi - no Grand Slam Cup
Connors - no French Open
McEnroe - no French Open, no Australian Open
Lendl - no Wimbledon
Sampras - no French Open
Edberg - no French Open
Becker - no French Open
Gonzales - no Clay court major (including Pro Majors)
Rosewall - no Wimbledon
Newcombe - no French Open
Courier - no Wimbledon, no US Open
Tilden - no Australian Open (don't think this is much of an omission - the Australian though existing wasn't a major until 1924 - note: He had the World Hard Court championships which is an equivalent to the French Championships - that is why I didn't mention the latter).
My view is that Borg lack of a US Open must hurt a lot as much as McEnroe's lack of a French, Borg having more chances of winning that tournament than McEnroe. I feel that Federer's current (At the time of writing) lack of an Olympic singles Gold isn't anywhere is serious as Nadal's lack of a WTF. There are two main reasons for that - the WTF is far more important (double the points) and also Nadal has had 4 times as many opportunities to win the WTF as compared to the Olympics. If you have 4 times as many chances to win something the lack of getting it is more glaring. Tilden lack of an Australian probably didn't even register with him. Sampras' lack of a French Open title hurts him personally, but his career is so amazing in every other category that it doesn't hurt him that much. Laver's lack of a WCT finals wasn't much (though he really wanted it) since he only got to compete in it first at the age of 33.