Hi there
To answer your question, there is simply no "best" force. Each have their own advantages and disadvantages. Dwarves for example have the highest defence values, good strength and fight values but suffer from poor movement and no cavalry. Elves have the best fight, courage and shooting values but suffer from defence and cost in points. Men are an all round force but suffer from mediocre fight and courage.
Now, the way I chose and started my own force was by simply picking an army I liked the aesthetic of. I encourage you to do the same. My forces include Galadhrim Elves and Rangers from the Grey Company list. I like the shape of the Elven armour and mysteriousness of the Rangers so I picked those. If you want to ally then use all three but start by picking the minis you like