The 36-year-old Somalia-born runner - who focussed on road racing after taking silver in the 5 000 metres final in London in 2017 - would have good reason to rate it that highly this year as he has to beat world record holder and defending London Marathon champion Eliud Kipchoge.
The 34-year-old Kenyan - who was given a good battle by Farah last year before the latter faded to finish third - is bidding for a men's record fourth win in the race on Sunday.
"My aim is to win the London Marathon one day," he said.
"That is my task, but you can't take it for granted because Eliud is a great athlete and a world record holder.
"If I do beat him it would be an amazing achievement."
Farah, who hinted in March victory in the London Marathon might prompt him to bid for a fourth successive world 10 000m title in Doha later this year, said his burgeoning rivalry with Kipchoge was good for the sport.
Athletics has found life difficult in terms of grabbing peoples attention post the retirements from the track of Farah and Jamaican sprint great Usain Bolt in 2017.
"Sometimes you are beaten by a better man and you have to accept that and you go home and think about what you need to do to beat him," said Farah of his defeat by Kipchoge last year.
"The rivalry is great for the sport - it's one the sport needs."
Farah has shown he is capable of winning major marathons having set a European record in winning the Chicago Marathon last October.
"I don't go out to try and finish third or fourth," said Farah said.
"I believe I can win more major marathons."