Haven't played since B/W...

What Pokemon game should I get first? Are any of them worth avoiding? Or should I play all of them?

Just play all of them, I think they are all worth at least checking out

BW2, then USUM imo. gen 6 isn't great and doesn't really introduce anything you'll need to know - even their main feature, Mega Evolution, has been shoved into the background/postgame in Gen 7.

Depends on what you like/want exactly.

If you liked Black/White, then definitely get Black2/White2. ORAS depends on if you're up for a Hoenn remake.

X/Y is.....if a poorly written plot and a barren post game is not a deterrent to you for whatever reason, then maybe it's worth a shot.

Don't bother with Sun/Moon, they're really not worth it, especially with Ultra Sun/Ultra Moon out.

I loved X/Y, new mechanics, a lot of new Pokemon to encounter. Also, the new Fairy type mixes up movesets and gameplay. The story is worth it and shiny hunting is easy in that game (breakable objects have a high rate, and swarms cut down on hunting time, not to mention the friend safari). End-game might be lacking (as mentioned above), but all Pokemon games get phased out by Nintendo anyways so I'm not sure that should be a disqualifier for playing the game.

ORAS was great for me as I loved the original version of the game and played the game so much the battery died. It's where I had my first level 100 Pokemon as well (Linoone). The Poke Radar is fun to use (and hunt a shiny you might want as well), and the Soar mechanic is how they should always let us fly around; It's so much fun.

The Sun and Moon have been great, even if there is a ton of story you have to sit through. I'd rather be running around battling than clicking next, next, next. USUM has lots of end-game stuff, but I haven't got that far yet.

Most of the games I like. Around 8 years ago I used to like Ruby/Sapphire but after recently playing it, I did not like the route and gyms. I have mixed rankings for Sun/Moon. I like the Pokemon and characters in Sun/Moon but at some scenes in the game I did not like. Red/Blue/Yellow, Gold/Silver, Black/White, and X/Y are my top 4 games.

I liked gen 6. What you need to know going into it though is that its almost the opposite of gen 5, it feels very reactionary as gen 5 was not like any game that had come before and not everybody liked the writing style or some of the more extreme designs. I do not know whether you are a fan of gen 5 or not, but I think how you feel about those games can be a factor in how much you would enjoy gen 6.

Unova was a more isolated region. The tone is dark and serious and it emphasizes character development above all. The story is also larger in scope than next gen with a challenging climax. Kalos is a more populated, colorful and energetic place, story is told at a much faster pace and keeps the content light and kawaii. These games look beautiful and concentrate more on novelty and world-building. Kalos plays it safer, there aren't as many new Pokemon but what they introduce is consistent and it buffs lots of old favorites.

While it is possible to like both games, you might also disappointed by XY if you really like gen 5, a common criticism is that its an easier game and doesn't have as much depth. However, I was able to enjoy both.

I think you should give XY a try. I would not recommend skipping entirely over gen 6 and heading straight from gen 5 to gen 7. Game mechanics changed dramatically between 5 and 6. Generations say 4 and 5 resemble eachother much closer than 5-6 do. While yes these games are designed for everyone to be able to to play them, if the world looked really different last you played it can make you feel more reluctant to connect to a new game that you might otherwise have enjoyed. I've played gens 1-6 and honestly I am still adjusting to gen 7, especially the art style and method of storytelling, and have not felt ready to complete these games just yet but I will eventually.

The thing about XY is that its a quick play through. If you don't like it then you can swiftly move on to something else without feeling like you've invested a lot of time in this game.

If XY spiked your interest in anyway then I highly recommend going on to Oras. Many people enjoy Oras, even it they are on the fence about Kalos. Its also a pleasant change of pace if you are a fan xy, where the imaginative innovations of gen 6 meet the pastoral settings and intimate relationships of Hoenn.

I'm not gonna lie, Oras is my favorite game. I am also a big fan of the original gen 3 games, especially Emerald, and think that GF made a wonderful retelling of the events of Oras, offering extensive development to previous characters, brand new characters and a vast, unexpected plot for postgame. However, because it is probably the most adventurous of all three remakes it is still not for every die hard fan of Hoenn. From making mega evolution a central part of the story to completely redesigning Mauville City, Oras really works to create new experiences in contrast to a remake like FRLG, which is almost more of a recolor than a remake.

If you are one-minded to some of the things I mentioned then I wholeheartedly recommend Oras.