Favorite undwear for toddler boy to learn to take on/off?

Visceral Mercenary • May you find your worth in the waking world.

My son just turned 2 two months ago. He's fully day trained so long as he is naked. But he has difficulty taking underwear on/off and ends up having an accident trying to do it. What brand and style was easiest for yours to learn to take on/off themselves? My eldest never had this problem but potty trained at 3, not 2.