Any of the Theme Decks from the Sun & Moon era (the last 19) are good. Of course, some are just a bit better than others. You're not going to have a bad deck if you choose any from the "top 10" (the most recent) in the game's Shop.
If you're going to mix two of the same decks together, trying to create your own Standard deck, just about everyone recommends 2 Mach Strike decks (with Garchomp/Lucario). I've never understood why others think that deck is great. Once you KO one or two of them, it's "game over." But that's just my opinion, because I think it's overrated.
For good cards to play, and promising Standard decks, you might try either of the Team Up decks that were released about a week ago. The Charizard one is awesome. And you could actually build two different Standard decks with it, either the Charizard line, or the Nidoqueen line. The Blastoise deck is good too, and carries almost an identical Trainer Engine.
Again though, any of the 10 most recent Theme Decks are good. They seem to get better and better with each new set.
My personal favorite (until last week anyway) has been Imperial Command with Empoleon. I just like it, nothing more. The secret, at least in Theme Events, is to be very familiar with your own deck (every card), and knowing the others, so there's no surprises. And then don't give up early on, which is very common. If you get paired with a Weakness to your deck (i.e. you're weak to their Energy type), that's always a hard game, and it can happen with any of them.
Edited by RobRatt, 08 February 2019 - 03:34 AM.