OnGuard Brute Mini U-Lock and get a cable for your front wheel as well.
If you’re gonna ride it as transportation and lock it up outside, just get a beater bike. Cheap off Craigslist, nothing fancy.
Definitely D-lock! Cable locks can be cut in less than a minute, while D-locks and chain locks take notably more time and that time difference can save your bike. Smaller D-locks are more difficult to cut because it's more difficult to get a grip on.
I've watched a lot of lock test videos, read reviews and ended up buying the Onguard Brute Mini Lock from Amazon. Paid $85 with delivery but maybe you can get it cheaper from other suppliers. The lock is quite heavy but it comes with a mounting bracket, so it's easy to carry on the bike.
It's a mount for a bicycle lock.
https://www.amazon.com/ONGUARD-45008112-Brute-Mini-Lock/dp/B007HOG61Y
Deal link: Amazon