Well, all players who I supported for years. I cried when Andre retired, still miss Moya and have crazy hope than El Mago returns once.
But Henin, mentioned higher, yep. Even if TOP-10 in ATP retires tommorow, plenty of intresting players will still be playing in ATP, it is worth watching. When Justine called it a day, the whole sport (WTA) for some time disappeared with her. Only later I understood how important for tennis and really talented she was.