I second that X is a solid choice. You will get a shot at getting a Kanto starter (ie Charmander, Bulbasaur, Squirtle) fairly early on in that game, and Pikachu can be found as early as Santalune Forest and Route 3 (about an equivalent length into the game as you would've found one in Red/Blue)
As someone said above, you
can get those starters in UltraSun/Moon, but I'm pretty sure you can't use the QR radar thing until after you've become Champion.
If you want to go full nostalgia, you can get Red, Blue, Yellow, Gold, or Silver from the 3DS eShop. They're exactly as presented on the original Game Boy, graphics and all. And you can eventually transfer the pokemon you catch in those games to other games using Pokemon Bank.
Since X and Y are the second most recent major games in the franchise, they also count as being good "modern" choices as well. Your other options for 3DS games are:
Alpha Sapphire and Omega Roby: Remakes "Generation 3" games Sapphire and Ruby from the GBA, retelling those games' stories with some tweaks and with modern graphics and gameplay changes which makes them, quite frankly, more playable than the GBA ones were. But being remakes of older games they are also kind of trapped in being more like their predecessors than the rest of the new batch of games, and will focus on the pokemon that were found in the original games, skipping large portions of the like 400 pokemon introduced since then.
Sun and Moon: the newest generation of games. The region they take place, Alola, in is based on Hawaii, which is quite cool and different than any of the other games. The games are also a little more "cinematic" than the others, with more emphasis on the story unfolding, which can be a good or a bad thing depending on how much you care about the story of the main games. There's also a big, cool sort-of nostalgia trip thing in their favor called Alolan Form pokemon. 10 classic pokemon species from Red/Blue/Yellow appear in the game with a twist: One or all of their stages of evolution are different from what you're used to in terms of design and type. For example, when you evolve a Pikachu in to Raichu in Sun or Moon, it becomes an Alolan Raichu instead, with a slightly different look and with a new type combo of Electric/Psychic instead of straight Electric. Or some pokemon are
entirely different, like Sandshrew. Sandshrew and Sandslash on Alola are Ice/Steel instead of Ground, with brand new moves that normal Sandshrews and Sandslashes can't learn. I know a lot of people around here have said that they don't feel these games have as much replay value as earlier games in part because of how slow they are to get going because of the cinematic setup. So that's a downside. here are some images of the Alolan mons I mentioned:
UltraSun and UltraMoon: The newest games. These are like the Yellow to Sun and Moon's Red/Blue. The story is much the same as Sun/Moon, the region is the same, most of the characters you'll find are the same. It's a game that feels very familiar if you play Sun and Moon first, but different enough to be interesting. And if you haven't played the other games first then it's samey enough that you shouldn't get lost or be confused. But at the same time, the same issues that dog Sun and Moon are probably present in full force here too.
I think you'd probably do well to get one of UltraSun/UltraMoon to be your "modern" game to X's more throwback-y type setup. And if you want full nostalgia, it's like $10 I think to get Red or Blue or Yellow on the eShop.
Also if you're ever confused about anything in the games, stop by the Pokemon thread on this board (not the pokemon Go thread though, we separated them) and we'll be happy to help.