TOS episode Space Seed or ENT episode Cold Station 12.

Thanks. I thought they were a TNG original, but now that I know, I will heed your advice

I didn't know Khan was an Augment. That explains the Eugenics Wars a bit

Yeah, true. If you're interested in khan, then watching TOS space seed first, followed by TWOK, then the other episodes might be the best way to get introduced to the augments (and khan specifically), to watch everything in the original release order (my preferred and recommended way), or you can start with the ENT episodes like borderland, or cold station 12 if you want to watch everything in the chronological in universe order.

I would recommend to watch the sources in order of release, as each episode and movie was made with an audience in mind that had the knowledge from the previeous ones. This would be:

TOS - Space Seed

TAS - The Infinite Vulcan

Movie II - The Wrath of Khan

DS9 - Trials and Tribble-ations

ENT - Borderland ff

ENT - Affliction ff

Movie 12 - Into Darkness

I'm pretty sure the eugenic wars are mentioned in TNG, too, but can't remember the episode(s). Also, everytime the Federation's ban on genetic engenieering is mentioned, this is at least a hint to the augment story arc. This would include Dr. Bashir in DS9 and Una in SNW.

Agreed with @Hurga on watching them in order of release, as all of the subsequent productions generally assume you're familiar with the ones released before it in a sort of wink-wink nudge-nudge kind of way, so you'll get more pay off from them watching them in that order.

Thanks. I will take your advice

Space Seed is the best introduction

KHAAAAAAN