Hi! I think this is a very open question with no simple answer so I'll do my best to try and help out.
The "best" cards can be very subjective and change pretty often! The best cards can be different for different people. A lot of the time the cards worth the most money are ones that are either very rare to get or are very common at high-level tournaments. If all the best decks at tournaments play 4 copies of a super rare card, everyone wanting to play in tournaments will want that card. An example would be
Ultra Ball from Plasma Freeze, as it's about 10 years old, new copies of it aren't being made, and tournament decks today are still playing it.
However, if someone just enjoys collecting Pokemon cards for the art, or because they have a favourite Pokemon they like to collect, something meant for a tournament player won't be very valuable to them. Paldea Evolved is a set that came out earlier this year with a lot of great art, so those kinds of people will likely find a lot of value in those cards. Something like
this Raichu would be considered better to them.
What card is the "best" really depends on the person collecting them. If you're starting to collect, I think a good starting point is to go looking for a few cards that you really like and want to have, and then find what set(s) those are from and go after those. Depending on if you're the kind of person who wants Ultra Ball or the kind of person who wants Raichu, you could build a totally different collection for the kinds of cards
you think are best.
If you're still unsure and want my absolute simplest answer, I think any of the Scarlet and Violet sets (Scarlet and Violet, Paldea Evolved, Obsidian Flames, and Paradox Rift) are where you should start. They're the newest sets and will be the easiest to find in stores.