Which season was Roger Federer's most important in his career? 
 2009 was arguably the most important of his career since he won his first and only French Open title, completing the career grand slam. He was also able to surpass Pete Sampras' record of fourteen men's grand slam titles.

Until then, Federer had won the three other grand slams - Wimbledon, Australian Open and US Open but the French Open was elusive as he had to repeatedly compete on the clay court of Rolland Garros against Rafael Nadal, who was a clay court expert and he lost to him on multiple occasions.

2004 was also an important year in Federer's career because he established dominance in the sport by winning three out of the four grand slams of the year and ending the year as the world number one for the first time in his long career.