"What is the best lense to buy for Canon Rebel T100"
The best lens for the Canon Rebel T100 is the lens that came with it. If you feel a need to try and better that lens, the very best option, really the only option, is the Canon EF-S 17-55mm f/2.8 IS USM Lens. Bare in mind it will cost a couple times what the Rebel T100 and kit lens cost by itself. If however you change your query from the "the best lens" to an additional compliment lens, then one of the 150-600mm super zooms would be great. Going to other way the EF-S 10-18mm f/4.5-5.6 IS STM Lens might be a good choice to expand the wider field of view.
EB
EOS 1DX and 1D Mk IV and several lenses!
