One of my friends has recently indicated an interest in the mobile Pokémon TCG platform that is TCGP, and has in fact started playing the game. Subsequently, that friend has also asked me for recommendations of where to start playing - specifically what packs to open and what decks to build. It's a fascinating question and one that I don't give much thought, since I've been playing practically from the very beginning. As such, now I'd like to consider the query and providing an answer to that query, for both the time that it was posed and the general consideration for when future expansions become released for the game.
For the current consideration, specifically at around the time of this post, the most recent expansions are of the B series that feature Mega Evolution. With that in mind, new and prospective players should try to make use of those specific expansions in order to build decks and battle. Depending on the deck and cards to play (or be played), however, one may need to utilize expansions from the A series in order to obtain possibly useful older cards that can still complement what is normally possible - even if it would become a matter of novelty and consideration for what decks could be manifested for current play of all kinds.
As for the general consideration, the current series is then always a good place to start looking for cards and building decks based off of them. This is especially true as the newer expansions always offer some refreshed mechanics in terms of the Trainer cards and some of the moves in the Pokémon cards, so going through them for deck-building inspiration makes for a good priority. After that, players should then feel free to look into the older expansions for cards that could change up the ways that they play, particularly ways that have been established by the prior experimentation of other players with those cards.
Still, there is also absolutely no harm in looking into all the existing pre-built decks across all the expansions, many of which can be said to remain relevant even in current times. This would also include the "Elite Decks" and their composition, even if it may not be possible to recreate that fully with currently present and obtainable cards. After that, then comes the challenge of making and implementing variations to them as any player of the TCG should do, whether the cards are virtual as in this game or its sibling TCGL, or of course with possession of the physical cards, however many or few there may be.
It's always a challenge for new players to start out in an already established game, even if that game has only been around for a year or so. That is (was) very much the case for my friend as above and the game of TCGP, who looked for guidance on the matter to me. With that, I thought I'd give my takes on the matter as I had presented them to my friend and now subsequently in a post on this blog. Whether that may or be helpful for others is up to them to determine, but based on my experience, at least a good deal of it should be. If so, then that achieves its purpose, and it'll be helpful for late players in making achievements in this game.
One year ago: Seeking for (My) Pokémon Unite Friends
Two years ago: The Values of Posts, Again
Three years ago: By Experience or By Information?
Four years ago: The Importance of Training in Café Remix
Five years ago: Realized Personal Social Media Experiments with Pokémon
Six years ago: Let's Talk About Events, Conventions, and Beyond
Seven years ago: A Tale of Two (OK?) Performances
Eight years ago: Ode to Magikarp
Nine years ago: Snorlax's Awakening