I've never played anything past Gen 3, and now that the games are all the way up to Gen 8, I'm looking to catch up. To do it, I'm gonna need a solid DS emu and some decent ROMs for the later games. What should I look for? What Should I avoid? Which games (Gen 4 - 7) are the best?

If you guys have any recs, I'd appreciate them.

 

The best DS games are hands down the remakes of Gold and Silver, Heart Gold and Soul Silver. Following those would be Black and White 2 in my opinion.

 

Desmume is a serviceable emulator; I never had any problems with it. I rarely play pokemon games outside of wacky rom hacks any more, so for the DS I would have to say my favorite games were the Ace Attorney adaptations and sequels.

 

I'm seconding HeartGold and SoulSilver.

If you're interested in ROM hacks, then I recommend Moon Black. Won't link it because I'm not sure about the content policy, but you could just google Pokemon Moon Black.

 

Blaze Black/Volt White 1 and 2. They're not super involved ROM hacks with massively changed shit but there's a lot of balance stuff and some type changes, movepool changes, evolution changes (no trade evos anymore) and all Pokemon are available. It's got a big difficulty boost too.

If you want monster changes and open worlds and stuff like that you'll have to stick with the gameboy advance ROM hacks unless the past couple years have added some new DS hacks.

You should be more than capable of finding them and instructions on your own.

 

It should be noted that Blaze Black and Volt White are hacks made by a guy called Drayano and they're specifically made to be much much more difficult than standard Black and White.