It depends on your yoga pants…and your overall comfort level vs fashion!
If you have thicker, higher quality yoga pants that are not going to suddenly turn see through when you bend a down, you will be fine wearing absolutely nothing underneath the yoga pants.
However if you are more comfortable wearing panties beneath your yoga pants, then a G-string will “look” the best. No visible panty line, etc.
The most comfy option will be to wear a pair of sheer, low cut short style panties, they might not look the best, but where they loose out in the looks department, they more than make up for it by being super comfy.
Another, option is to wear a pair of crotch less Yoga Pants. The ultimate in feeling sexy and free, but remember to wear them only in the privacy of your own home! As not everyone wants to see you actually feeling sexy and free to that extent in public. : )