Happiest: well, there are a few. The obvious ones, like the Slam wins, the Hingis win at the YEC in 1997, the 2000 Roland Garros Doubles win, the 1998 back-to-back titles in Moscow and Luxembourg, beating Capriati (at last!!) in Filderstadt in 2003, and of course the fairytale run to the final at Roland Garros this year.
For me, one of the saddest moments was Mary's inexplicable loss to Dokic at the 1999 Wimbledon. Mary was on good form throughout the tournament, Hingis had been removed from her path, and it should have been a fairly easy ride to her second career Quarter-Final at Wimbledon (against Alexandra Stevenson!!!). She came out all guns blazing against Dokic, leading 3-0 and 4-2, and then...she fell apart, and Dokic won a lacklustre match. It took me ages to get over that loss for some reason, and it also, perhaps unfairly, made me HATE Dokic for years.