Well, the #1 most environmental way is to simply not consume coffee (shipping the beans from there to here and processing them produces a lot of emissions), but I’ll assume that’s not an option. Reusing your own cup is certainly better than using a disposable one every day. Get whatever one suits your needs best. I have a tall mug here at work (I use it for water, but it could be used for coffee), but I would use a travel mug if I were getting my coffee from a cafe. For further eco-friendliess, buy or make a coffee cozy, a reusable cup sleeve is better than even the recycled cardboard ones they offer. Plus you can get one with cool fabrics, versus the plain brown bleh logo cardboard.