Warming up breast milk

What’s the best bottle warmer that can fit all types of bottles out there. I need help ladies. Right now I’m breastfeeding on demand and if my baby sleeps extra long that’s when I pump two bags or three maybe or if we’re going out and I’ll be long I pump before going out. But I’m going back to work soon so I’ll be leaving my breast milk with my SO mother since she’ll be watching the baby. And want to buy a bottle warmer to make it easier since she’ll be watching my one year old too.