My S'well 16 oz travel mug is wide mouth, steel and double layer insulated. Cleans easy enough and I got it on sale. I know S'well is expensive, but they do last forever.

If you want to upcycle and super easy cleaning, get a protective cover and drinking lid for a wide mouth mason jar.

But bottle brushes! I bought a set for not much on Amazon, and they clean everything. If something looks stained or dodgy I put a denture cleaner tab in overnight.