How do I know I'm choosing the best card in the keg?

In Gwent, when you open a Keg you get to choose which rare you get among three different cards.

If I'm not aiming for a particular deck (or if none of the cards I get are in that decklist) how do I know which cards I'd better get? I'm not an experienced player, I've seen a few decks and I have no idea if they're strong or if I just played badly against their startegy so I have no idea which cards are good, or if there are ostensibly bad ones I should avoid.

I'd like this decision to prove valid at least for the current meta.

There are four different card raritys at the moment:
Grey is "common", blue is "rare", purple is "epic" and gold is "legendary".

The last three cards of a keg, from which you can choose one, are always of the same rarity, so if you don't see one that has an effect that you know is of use to your current decks, it doesn't matter which one you choose.

There are no "bad" cards per se, the mixture in your deck only makes them more or less useful.

A good way to choose your last card is to look at its color. In a deck you can only put 3 of each bronze, 1 of each silver and 1 of each gold card, which means that duplicates of silver and gold cards and fourth bronze cards are useless. The only use for these duplicates is to mill them in your card collection overview to get some scrap. There is a "mill spare cards" button that will automatically scrap your duplicates or fourth bronzes. If you have the choice between a new usable card or a duplicate or fourth bronze, always choose the new card.

If you happen to get the choice between three legendary cards, see if the portrait of one of the card is animated. In the most recent patch they added these animated versions for gold cards, which have the same effect as the usual ones but cost double the scrap to craft them individually (800 for the normal card and 1600 for the animated one). They can't be milled for more, but since the animated card is more expensive to make, it's a preferable choice in my opinion.

Ultimately, which cards you choose simply comes down to which card effects you desire. Since you don't aim for a particular deck you can just look at the card effects and think about if they sound useful to you. I'd recommend that you have a look at the Card Collection to see which cards there are and try to remember when an opponent plays a powerful card or if they use an interesting strategy so that you can try to collect specific cards and build a strategy of your own. There are definitely no wrong choices or bad cards.