The tax angle folks haven't touched on is huge - most countries with digital nomad visas don't tax you on foreign income during that visa period, but the US still does since you're a citizen, so you're filing taxes in both places and potentially dealing with FBAR requirements depending on where you park money.
That said, Mexico and Portugal tend to have the most straightforward programs with decent infrastructure for remote work, though honestly the visa is almost secondary to whether the place actually has reliable internet and a timezone that doesn't wreck your sleep schedule.
Sounds like others already covered verifying current requirements, which yeah, that's critical since these programs shift every year or two, but don't sleep on the tax situation because that's where people get surprised later.