I love the Pokemon series! I myself started with Sapphire, but by I think that by now I've played almost every single one of the handheld games (GBA/DS).
My favorite game has to be Pokemon Black, but the old games have something about them that always make me want to play them again and again.
Yeah man, old but gold! I've played Black too and its pretty decent! I was going to give Pokemon Moon a try, is it worth it?
I personally enjoyed Sun & Moon but some people don't really like them... I guess it really depends on the person.
They're far more easier than a lot of the other Pokemon games but that's not necessarily a bad thing... Again, it depends on the person. I did really like the Z-Move mechanics they added, mostly because the dances look silly.