Most common organism causing T. capitis is –
"Ringworm of the scalp occurs in children exclusively and is mainly due to M. canis".
Following statement of Harrison creats some confusion here :-
"The predominent organism is T tonsurans".

Therefore, I had to do extensive search on this question and I collected following information :
Most common organism causing Ectothrix is M. canis, whereas most common organism causing Endothrix is T tonsurans. Ectothrix is the most common type of T. capitis, therefore M. canis would be the most common cause of T. capitis.
Trichophyton tonsurans
Microsporum
Epidermophyton
Candida albicans