Tough to say, as the mechanics have had tweaks over the years and as such, while it's mostly the same game, there are a number of differences from Gen 1 (Red/Blue/Yellow) to Gen 7 (Sun/Moon/Ultra Sun/Ultra Moon).
For practicality's sake, if you want to get an idea of what Pokemon is like currently, any game from Diamond/Pearl/Platinum up to Ultra Sun & Ultra Moon is good. So that gives you Diamond/Pearl/Platinum, Heart Gold/Soul Silver, Black/White/Black 2/White 2, X/Y, Omega Ruby/Alpha Sapphire, and Sun/Moon/Ultra Sun/Ultra Moon to choose from.
See, there are 18 types of Pokemon and in the first three generations (1: Red/Blue/Yellow, 2: Gold/Silver/Crystal, 3: Ruby/Sapphire/Emerald/Fire Red/Leaf Green) whether an attack was Physical or Special was determined by the Pokemon's type. So, back then, a Fire type move would always be Special. But Generation 4, Diamond/Pearl/Platinum, introduced the Physical/Special split and now attacks can be either Physical or Special individually instead of being restricted by the type. This is better because you used to have Pokemon with a high Attack stat that might have only learned good Special Attacks, or vice versa. Now more Pokemon have a chance to flourish in their own niche.
I would probably recommend Pokemon X & Y as a good start because, compared to other games in the series, it is a bit easier. It is also a game you can still utilize the online features of so you can trade with people. Sun/Moon/Ultra Sun/Ultra Moon are the most current but they are outliers from the rest of the Pokemon games because there are no Pokemon Gyms. See, in every main series game of the series except for the most recent ones, you challenge the Pokemon Gyms found around the region in which the game takes place, ultimately face the leaders of said Gyms, and if you defeat them you earn badges that show your accomplishment and permit you to train Pokemon above a certain level without them starting to disobey you. Once you collect all 8 badges of a region, you face the Elite 4 of the region and attempt to defeat the Champion of the Pokemon League.
Alternatively you could get Red, Blue, Yellow, Silver, or Gold from the eShop on the 3DS for much cheaper than one of the DS or 3DS games and experience things like many 90s kids did when Pokemon first came out. I'd still advise against it for the mechanic differences I mentioned above but at the same time, if you're going to enjoy those games, it'd probably most likely be if you played them first and saw how much things have improved over the years. I won't speak for everyone, but I and a number of other people feel that the R/B/Y/S/G games are too archaic at this point, so for whatever that's worth. I'd stick with the X & Y recommendation because if you get used to that, there's nothing to have to get used to all over again if you enjoy them and want to pick up other current Pokemon games.
I hope this was helpful and that such a long post wasn't daunting. I'm simply a long-winded person and get carried away sometimes. Whichever game you might choose to start with, I hope you enjoy it.