What is best fabric for the leggings(sportswear)?

You are on the right line in terms of the  fabrics -poly + elastane or nylon+ elastane. If they are meant as active wear or sportswear -I would suggest to get the fabric to have a wicking finish (which will draw sweat away from the skin) and make the wearer feel cool.

If your market can afford actual lycra (instead of generic elastane) then obviously that is better. Invista which does lycra also does wicking, the brand being Coolmax.

Be careful to check out the weight and thickness of the fabric you are using - that it is not transparent. When stretched,  as would happen say in exercising, fabrics gets thinner and more transparent and as luck would have it it happens more on the buttocks!!

If the leggings are for just casual or street wear, you could also look at cotton or rayon blend with a heavier weight.

when you have the leggings stitched -take care of the seams - they need to stretch but also should not be rough as they will be tight against skin and rubbing