I'll grab links in a second (and there are some good articles I'd like to find that gather lots of evidence in one place), but here's some stuff off the top of my head:

Wikileaks published a private diplomatic cable stating that no one was killed in the square itself, although a smaller number of people did die in clashes elsewhere in Beijing, consistent with China's own official account. (Here is the cable)
A spanish film crew was in the square all night and filmed crowds of people walking out of the square in the early morning, singing the Internationale. Two wounded people are shown among them, possibly people who had been brought to the red cross station in the square, but no gunshots are heard, no one is running, and there are no bodies. (courtesy of /u/robinn, here's footage of a Hong Kong news report that includes the spanish film crew footage)
two of the main organizers of the protest, Hou Dejian and Liu Xiaobo, state that no one died in the square, and call out other organizers for lying (courtesy of /u/GarbageShoot: interview where Hou Dejian, a Taiwanese national and one of the leaders of the Tiananmen protests, says he was in the square all night and saw no one killed, and Liu Xiaobo agrees with him — and also courtesy of /u/robinn, here is a twitter thread covering testimony by various organizers, including Hou Dejian and Liu Xiaobo)
Numerous western media sources have stated that no massacre occurred in the square. (This article links to multiple western sources, including James Miles, attesting that no one died in the square. — be warned that this site also hosts some crank articles, you might want to focus on the sources)
various western massacre reports cite wildly different death figures, usually with little or no justification for the number
an attempt to collect all the names of the massacre victims ended early when they only found 155 https://en.wikipedia.org/wiki/Tiananmen_Mothers
CIA and NED goons were known to be present in Beijing and involved in the protests. (Here is an article from the Vancouver Sun in 1992, showing western intelligence involvement was known in the west decades ago)
during most of the protest, protesters were calling for a return to stricter communism, not for liberal market reforms. These were Marxists. Their signs showed Marxist figures and slogans. (This article shows some images of the protesters displaying Marxist slogans and iconography and discusses it a bit — again, be warned that this site hosts some crank articles, so you might want to focus on the sources)
toward the end protest signs were suddenly in English
tank man: the guy survives and is led away by bystanders, and also the tanks in the video are leaving the square (you can see this in the uncropped footage) and it is broad daylight, whereas the main violence occurred at night (also, courtesy of /u/robinn again: China in 1989 for one man vs. the U.S. in 2020 for a whole crowd and courtesy of /u/LegaliiizeIt, another video in the same vein)
online photos of corpses were visibly taken elsewhere, not the square
the first violence was against troops, not civilians. On June 2, 1989, two days before the June 4 incident when the main violence occurred, multiple unarmed Chinese troops were burned alive and their corpses hung from nooses in public. ((CW: gore) courtesy of /u/robinn, here is a thread of photos showing dead and wounded troops, some being rescued by civilians. Multiple men were burned to death, others were beaten. Some protesters stole guns from the army and can be seen brandishing them.)
the violence against troops was uncharacteristic of the previous tone of interactions between troops and protesters in the preceding weeks. Troops and protesters had peacefully coexisted, singing songs and sharing food together. (Here's an article that goes into it a bit — again, be warned that this site hosts some crank articles, so you might want to focus on the sources)

I don't really have much off the top of my head for Uighur genocide stuff

libs should hear who Zens is
even the US State Department and the UN concede there is no mass killing in Xinjiang.
western intelligence has been stoking and funding islamic extremism in Xinjiang. There's that talk by some American general or colonel where he acknowledges this, and I think there are other sources online as well
/u/robinn made this card with lots of Uighur info
Qiaocollective is a good source on Xinjiang, courtesy of /u/BynarsAreOk

I'll stick articles down here, some of which aggregate lots of evidence in one place

How psy-ops warriors fooled me about Tiananmen Square: a warning ...edgy-sounding title though

1989: Tiananmen Square ‘massacre’ was a myth

How Much Longer Do We Need to Propagate Lies About Tiananmen Square?

Tiananmen — The Big Lie

Tiananmen: The Massacre that Wasn’t

1989 Tiananmen Square "Student Massacre" was a hoax

The Truth Behind the Myth of the 'Tiananmen Square Massacre' - Opinion Piece By Dr. Dennis Etler

Notes for 30th Anniversary of TianAnMen Incident, courtesy of /u/GarbageShoot

Tiananmen Square Massacre – Facts, Fiction and Propaganda Lots of good images and western media quotes, but the site hosts some goofy articles so be careful citing it

Let’s talk about Tiananmen Square, 1989: My hearsay is better than your hearsay

Birth of a massacre myth, courtesy of /u/GarbageShoot

CBS news: There was no "Tiananmen Square Massacre", courtesy of /u/robinn

a NYT questioning the massacre, courtesy of /u/robinn and here's a pastebin rip

dessalines, a Lemmy dev, has a good socialism FAQ, including sections on Tiananmen, courtesy of /u/PorkrollPosadist

[link] [link] these two /r/ChangeMyView comments on China are great, courtesy of /u/geikei

[link] [link] [link] Some old hexbear threads, courtesy of /u/Finger

not China, but /u/robinn also made a carrd on the DPRK and linked a good twitter thread by ProleWiki

Comments in this thread to check out. I'll try to also add their info up here.

/u/robinn: https://hexbear.net/post/271448/comment/3513792

/u/Awoo suggests keeping things short and digestible instead of posting long effortposts, since libs will just count on people not reading the whole thing and they will point at it and say "See? They deny the Uighur genocide" or "See, they deny Tiananmen" https://hexbear.net/post/271448/comment/3514089

/u/GnastyGnuts has some great links but I hit char limit https://hexbear.net/post/271448/comment/3514972

/u/Krause has more great links https://hexbear.net/comment/3529874