The Spaniard is set to re-join McLaren after confirming his Ferrari departure at the end of last season, with four-time world champion Sebastian Vettel filling the void.
Despite winning two world titles with Renault in 2005 and 2006, Alonso insists he was at his "best level" during his largely unsuccessful time with Ferrari.
"From one to 10 I think in those five years I will put nine," he is quoted as saying by Autosport. "I know it seems high, but I have enjoyed it so much - I grew up as a driver, as a person.
"I have worked with some amazing people here, and I raced with my heart, I raced at the best level of my career."
Alonso has relished the challenge of going up against team-mate and former world champion Kimi Raikkonen this season and is delighted with how he fared.
"Even this year, I have a world champion on the other side of the garage and I had some doubts, but I was curious to see the performance and what I could learn, what I could improve," he added.
"I got only two podiums, but [versus Raikkonen] I was 16-3 in qualifying, 17-2 in the races.
"I never imagined to have this beautiful performance over these five years, fighting for the championship, racing with everything I have inside, never giving up.
"I am proud of the team and of the things we did together.
"We missed the title, but we raced against a very dominant era of F1 with Red Bull, and this year with Mercedes - no one could get closer to them.
"This is what I missed to get to 10 [out of 10] for these five years, but any driver should drive for Ferrari for one year because it is a unique spirit."
McLaren will confirm their 2015 driver line-up on Thursday.