My name is Estro Jen, from LA's Angel City Derby Girls and Moxi Skate Shop in Long Beach. The very best stops are called Snyder Stops. They are made of a gummy rubber and grip so well you can bounce up and down on them and get rebound. They come in three sizes, dance plug (best for banked track starts), Medium (best for flat track roller derby) and Large for artistic skating. They are a bit pricey compared to other stops. ($27.) but they are TOTALLY worth it! They make it so much easier for beginner's to learn and are the most reliable for advanced skaters. When looking for toe stops, grippy is always best ;)
btw- I wouldn't suggest ever using them outdoors- they get torn to shreds.