For me it's Adobe After Effects. Yeah, I can do most of what it does in a combination of blender, natron, gmic, etc.. but I really like the workflow of AFX. Not having that tool was one of the hardest parts of cancelling my Adobe subscription. Nowadays I would even settle for a non-foss alternative. As long as it's running on Linux. But so far, that has not happened (I use other non Foss tools that work great, like resolve/fusion and Houdini.. but I still miss AFX)
Edit: yeah, I missed a detail in the question: I do not currently use AFX but used it a lot in the past and am now trying to replicate workflows I based on it with other tools.. still miss it a lot and would give a lot to have a solid alternative...
I often recommend people using the "100 searches" trial of kagi to see if it might be worth it for them. I tried it and for me the price is worth it because of the custom filters and weights for results. It's really nice to search for something and all results on the first page are relevant content (no ads, seo-bait pages or stuff like Pinterest).