As of now on the beta, 110 is looking like it's going to be the level to twink at. Reason being, dungeons currently drop baseline 310 iL gear for 120s, but the gear requires 110 to use. A full 100 iL above 110 dungeon gear. These pieces include Azerite pieces. The only pieces that wouldn't be 310 pieces would be Mythic ABT trinks since BfA dungeon trinks are hot garbage, and ofc your two legion legendaries.
Chances are however, this won't make it to live. In that case 111 will be the level to twink since there are iL 300 crafted pieces that require 111 that have a chance to upgrade to 310 upon being crafted. In this very likely event, your BiS set would be as follows:
1. Your 3 Azerite pieces that you receive from completing each of your faction's zones. (Just like the 800s from completing Highmountain, Stormheim, etc.)
2. Whatever crafted 111 gear is available to you. (I wouldn't recommend trying to get pieces from a different armor type if you're lacking in a certain slot as the benefits of that will not be the same as 101 gear in Legion.)
3. Mythic ABT gear in whatever slot is not an Azerite piece or crafted piece. You might actually even be able to use Mythic ToS gear, but we'll have to wait until character copy is implemented to have an idea of which is better overall.
4. Last but not least, your two legion legendaries. This may be reduced to 1 but I'm not sure on that, have to wait and see if order hall upgrades are still in effect come BfA.
I hope at least one thing explained in this was useful to you or anyone else that has questions about twinking around this level come BfA.