I gotta say, ORAS is my favorite so far. It may seem a little bandwagon with it being the newest game and all, but it's my favorite! Pokemon sapphire was m first Pokemon game, and it taught me many things.
I focused on only training my mudkip and neglected my entire team! This game taught me diversity and the importance of equalty. I needed to equally level all my Pokemon to actually complete the game, If I didn't do so, I couldn't get past the 5th gym. I also needed diversity in Pokemon types to help beat certain gyms. It also taught me simple math with all the stat boosts and leveling. Overall, it was good for me, and I learnt much from it. Alpha Saphhire was a giant hydro pump of nostalgia for me, along with the delta episode. It had all the great features from x and y, and it brought back secret bases! I'm especially glad my favorite legendary, Rayquaza, had such a spotlight in the end. I feel like the delta episode is supposed to be a sneak peak of an emerald remake, but even if Emerald isn't remade, I think the delta episode covered enough for that.