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What chapter (not grey knights) would have the most numerous and/or best psykers? Wanted to paint loyalist thousand sons but realized that it'd be hard too make, so I decided to find the loyalist equivalent of a "psyker legion".

That is exactly what Blood Ravens are for. Loyal psyker marines with a touch of maybe being 1K Sons descendants.

I dunno if they'd nesscarily have the BEST (the best loyalist astartes psyker over all is proably Tiggy or Mephestion) but proably the chapter best known for it's psykers would be the blood ravens.
https://warhammer40k.fandom.com/wiki/Blood_Ravens

Blood Angles

They have a lot is Psykers. Mephiston is a beast. One of the chapters that the Deathwatch (along with Blood Raven) with get the most of their Librarians from. A librariaius founder. Etc.

Also Grey Knights.

I never played DoW games so I never cared much for Blood Ravens, and the conspiracy theory of Secret Traitor geneseed in any Chapter seemed like wishlist fanboy headcanon.

But to OP, Blood Angels get the unique Dreadnought Librarian entry, which puts them ahead of the others.

Why is any secret traitor geneseed automatically fanboyish?

In terms of most common psykers, the Blood Ravens (note that they don't have EXTREME amounts, but just noticeably more). Them being ex-Thousand Sons may or may not be true, so it's best not to bank on it.

As for the best psykers? Blood Angels, almost certainly. Mephiston is almost certainly the strongest psyker Space Marine (maybe aside from Grey Knights and Chaos) out there, and the rest of their Librarius is similarly quite powerful.

However, if you're after a "psyker Legion" aside from Grey Knights, that's not really thing on the loyalist end. There are certainly ones that favour psykers (Blood Angels, Blood Ravens, Silver Skulls, even Exorcists), but there's none that quite do it to the same degree as the Sons.

Blood Angels were the second most psychic of the Legions, and that still persists.