It really depends what you’re after. If you’re looking to make a profit you’ll probably want to stick with cards which get talked about a lot (like the Masaki promos, Trainer Certification Card and Pokémon Card Laboratory mentioned above).
If you’re in it for the rarity aspect I’d recommend looking at Battle Road promo cards - a lot of those are still incredibly cheap in relation to their overall print run. As an example, a PSA 10 149/PCG-P Rainbow Energy sold on eBay last year around the same time all the Logan Paul hype was going on for just $39 despite it only having a distribution of around 200 cards. If you’re interested I’d advise checking out the Master List of the Rarest Pokémon Cards - that attempts to list all cards with a distribution under 1,000 and a lot of them are readily available for very little.