The best mutual fund for long term investment is a diversified mutual fund. Within that the best sub category would be a multi cap fund with flexibility to invest in mid or large cap funds based on the situation.
I am not calling out a small cap fund, as the best fund here because the returns for small cap funds are volatile.
From an expense ratio standpoint direct plans have the least expense ratio. The difference is 1% to 1.5% which means that in the long run, you can end up with 40% extra in your kitty.
While this status keeps changing with time, two good funds are Invesco India Contra Fund and Aditya Birla Sun Life Advantage Fund. Choose a platform such as jamawealth.com that offers only direct plans for your investments.