‘I will do my best’ – perhaps is the worst articulation of your abilities.
Never ever say this in a professional realm. I have heard it time and again (and have even used it without realising the kind of image I was projecting). It’s a horrendously misleading phrase and it creates more of a negative connotation than a positive one. If you are required to finish 5 reports and you finish just 2 of them, you can say ‘I did my best’ (and you probably did) but did you complete your task? No. By no means. Don’t say you will do your best because your best may not be enough!
Doing your best is not enough. Finishing the job is. And by saying ‘I will do my best’ does not show your abilities and skills but instead it limits them terribly. Prefer saying ‘I will get the job done’. That sounds professional, definitive and assertive. Push your best to the limit and finish your work. Because your best should not define your work, your work should define your best.
Usain Bolt could run a 100m sprint in 20 seconds and say he did his best. That certainly won’t get him the medal. His best has to match (or better) what’s required of him and that is under 9.6 seconds.