Honestly, Pokemon is a series that you can start from anywhere. If you didn't know, Pokemon games are generally released in pairs with some Pokemon available only in one game and some only available in the other, to encourage trading. Sometimes that set of 2 sometimes gets a 3rd game, which typically adds more things. Here's the pairs/trios.
Red/Blue/Yellow
Gold/Silver/Crystal
X/Y
Alpha Sapphire/Omega Ruby
Sun/Moon/Ultra Sun/Ultra Moon (they did a pair of the two Ultra games instead of a 3rd version)
Generally you can just get away with only buying one game out of each set and just trade for any Pokemon that are unavailable in your game. But something to keep in mind is that you can't trade or battle online for any of the Virtual Console Pokemon games (Red/Blue/Yellow/Gold/Silver/Crystal). If you want to do online things, you need to buy X, Y, Omega Ruby, Alpha Sapphire, Sun, Moon, Ultra Sun, or Ultra Moon.
I recommend skipping Sun/Moon and just going for one of the Ultra games if you decide to get any of those. The Ultra games add in a bunch of new things so it really isn't worth buying the original Sun or Moon anymore.
You didn't mention:
Original Ruby, Sapphire, and Emerald (though those are GBA exclusive and not on the eShop Virtual Console currently, so obvious why that was omitted)
Diamond, Pearl, and Platinum
HeartGold and SoulSilver
Black, White, Black 2, and White 2
As for what I'd recommend, if you want something that is still somewhat current that you can enjoy fully with online functionality and a lot of content you would probably be best served going with
Omega Ruby or
Alpha Sapphire. Not only will they give you the idea of what a traditional Pok?mon game is like, but they offer a more challenging and thus more rewarding experience than X & Y.
Alternatively, since
X &
Y were also for the 3DS, they too still have online functionality so you can trade and battle people but offer an easier, more laid back experience so you can get your bearings and take it slow to learn things. These are probably more new player friendly because of that, and might make for a better introduction to the series, but honestly Pok?mon is not as complicated as people make it out to be. Even the seemingly complex aspects are simple when one puts the time into learning about them. One advantage X & Y have over Omega Ruby & Alpha Sapphire is Trainer Customization, which means you can choose the skin color of your trainer for the first time in the series, buy different clothes for your trainer and change their haircut, hair color, and eye color. This feature did not return in Omega Ruby & Alpha Sapphire since they were remakes of older games, but the feature did return in Sun & Moon and Ultra Sun & Ultra Moon, and will presumably be in all non-remake Pok?mon games in the future. Also, each copy of X & Y give you access to the original starters from Red, Blue, and Yellow which are the ever memorable Bulbasaur, Charmander, and Squirtle. You're able to choose any one of those you want, and the others you don't pick you can get from other players online. If you like any of them then that is something to consider.
Anything older than X & Y and Omega Ruby & Alpha Sapphire either never had or no longer has the functionality of being able to trade over wi-fi. Diamond, Pearl, Platinum, HeartGold, SoulSilver, Black, White, Black 2, and White 2 all used to have online functions but they were discontinued when the company Nintendo (and other companies) used for servers closed down a few years ago. They are all solid games and worth playing, but you won't get the full modern Pok?mon experience if you start out with them.
Once you get your bearings with whichever game you pick, I highly recommend HeartGold and/or SoulSilver because they feature 2 regions, Johto and Kanto (from the original Red, Blue, and Yellow), and that means 16 badges to collect instead of 8. It also has a ton of fun features like having the first Pok?mon in your party following behind you outside of its Pok? Ball, the Tuesday, Thursday, and Saturday Bug Catching Contests, and the Pok?athlon Dome which is a sports arena where you control your Pok?mon as they play sports-themed minigames. The only downsides of these games are the inability to play online these days and that they were quite popular and are more expensive than other Pok?mon games because of it.
As for Black, White, Black 2, or White 2 games, you should go Black or White if you want a better story and Black 2 or White 2 if you want more locations that weren't present in original Black and White, a few new features like Pok?star Studios, and access to legendaries from previous generations.
I've never really seen much special about Diamond, Pearl, and Platinum so I wouldn't recommend them. Not wholly because I find them bland though. They will likely see remakes in the coming years, likely after the next main series installments in Fall of 2019, as they're the next games due for the remake treatment.
As far as the Red, Blue, Yellow, Gold, Silver, and Crystal ports on the 3DS eShop Virtual Console are concerned, I find anything older than Diamond, Platinum, and Pearl to be too archaic by today's standards to really enjoy. There have simply been so many quality of life improvements to the series over the years that those games are only good for feeding nostalgia. Furthermore, because of certain quality of life improvements like the physical/special split introduced in Diamond, Platinum, and Pearl, playing anything older will just make you more confused as to how things currently work.
As for the most recent generation, Sun & Moon and Ultra Sun & Ultra Moon, I wouldn't recommend them as first games because they lack the traditional gym system. They are fun, but they will not give you the proper impression of what the series has always been. They're worth playing, but not first.
But yeah, I'd say either Omega Ruby or Alpha Sapphire for a fulfilling experience that still has modern conveniences or X & Y if you want an easier time learning, access to Bulbasaur, Charmander, or Squirtle, and Trainer Customization, plus the same modern conveniences.