You should start with the one that interests you the most honestly, at the end of the day you are the one playing and all Pokemon games are good to start with.

Buuut, if you want to start with something easy, you should start with X/Y.

Fire Red, good old pokemon red but with better everything

XY. It's more accessible, it's prettier, has the best online (alongside with ORAS), etc. Perfect for a starter.

Unless you really care about story (something that is not a strong point in this series), just stick with this generations' games. You'll have access to every single Pokémon legally released, you'll be able to trade and battle others online, and you won't have to get used to different mechanics over again going from one generation to another.

First, DO NOT start with the 1st or second generations made for the Game Boy and Game Boy Color. The remakes of these games on the GBA and DS are superior options, especially for newcomers. Also, DO NOT play something like Black/White 2, which is the oddball direct sequel.

I feel that generally, out of the DS and 3DS games, your best bet would be X/Y or AlphaSapphire/OmegaRuby. Along with being the most recent and thus polished games, they are successful at what they try to do.They both also contain the best mix of Pokemon to capture and battle.

I feel that X/Y is a bit better for somebody who wants to explore an in-game world for the first time, and contains better side content. AS/OR has more late-game content, and a better story in terms of enhancing the game's flow, but takes place in a less interesting setting for first-time players.

I would not reccomend Diamond. Pearl, or Platinum because these games move at a fairly slow pace without much benefit compared to other entries. I personally love Black/White, but those are best played after other games because of their specific themes in terms of design and storytelling. HeartGold/SoulSilver is full of content, but lacks the sophisication of more recent games.

just start whit X/Y... don't go whit fire red/leaf green, their battle system is outdated whit things like ( water always SP.attack/ flying always attack, etc...)

start from the games from generation IV (diamond/pearl and heartgold/soul silver.

From a story perspective, it does not matter which one you play first. The only exception is that you should play Black/White before Black 2/White 2. From a difficulty perspective, XY or ORAS first. Those are the easiest. From a gameplay mechanics perspective, don't touch anything before Generation 4. And then instead of Diamond/Pearl, go with Platinum. Save Generation 5 for last because you will more appreciate its deep story if you do. I would say start with ORAS, XY (unless you would rather wait for Z), HGSS, or Platinum. Then try Black/White followed immediately by Black 2/White 2, and make sure the sequel you get is the same color as the first one.

However, if you simply want to play the greatest Pokemon games ever, go with Colosseum, followed by XD: Gale of Darkness.

P.S. From the OP, I assume you have a 3DS, which means you can play everything from Generation 4 onward. Do you also have a GBA or a DS (OG or Lite)? Because if not, there is no point in anyone advocating you start with anything before Generation 4.

I would also like to start playing a Pokémon game for the first time. But I want to narrow it down to the recent ones. Should I go with X/Y or Omega Ruby/Alpha Sapphire?

Story is really not a thing in these games except until you get to XY. Start as far back as you'd like, but note that Red/Blue haven't aged well.

One strategy would be to start with ORAS so you could skip XY and go straight to (the theoretical) Z, but besides that, I don't think it really matters.