I was on the hunt for a reliable tea travel mug for the longest time. I was being too picky with each one - either they were too small, not spill-proof or not very ergonomic for work. J ended up finding this 16 ounce Leak Proof Drink Bottle with Tea Infuser from Thermos on Amazon and it has honestly been a dream to use.
What's great about this mug is that it does an INCREDIBLE (no exaggeration) job of keeping your drink hot. The mug was advertised that it would keep the drink hot for up to 12 hours or cold for up to 24 hours. I haven't tested out the latter but my drinks stay piping hot when I use this bottle. During my first usage, I poured boiling hot water into the mug and nearly 14 hours later it was still very warm.
One thing that I am a little worried about is the actual tea infuser. You kind of just pop it in and out and I worry that as time passes, it will not pop back as easily and end up coming out. Despite that very minor feature, this is my go-to mug for work now. I cannot say enough good things about it.
You can purchase the mug HERE.