So. many. random. items! I can't pick just one.
Turkey baster is definitely top of the list. I got one to be able to move cory fry from container to nursery tank when they hatch, and also to clean out their tub - so useful for those things!
I also use it to drop food into the dish I feed my shrimp from, and for spot cleaning tanks, like removing bits of leftover food tablets without having to start a syphon and drain water etc.
I also have to have to system to carefully check buckets after a water change for shrimplets, and for tiny, almost invisible pygmy cory fry. Means emptying the bucket jug by jug until there's only a little water remaining- and a whole lot of muck, where the fry blend in perfectly. Turkey baster comes out to catch any I spot and return them to the tank, and to gently puff areas of the muck and check for fry. Then to remove the muck one baster full at a time into a jug, to look and make extra sure there are no fry in there. Takes ages, but has saved many a baby, and the turkey baster is perfect for the job.
Ones not mentioned yet: Stanley knife blades are great for scraping stubborn algae and hard water marks from glass, and for freeing stubborn suction cups.
I use a dishwasher brush like this one for all sorts of things;
Scrubbing algae from driftwood/stones/decor, scrubbing new wood/stones/decor, cleaning filters and stuff, whatever it comes in handy for.
One drawer in my fish stand is dedicated to fish only cleaning stuff, all sorts in there!
Oh, bottle brushes of various sizes for all the reasons listed above, including an extra long double headed one for cleaning canister filter hoses out. They collect a lot of yuk that would get blown back into the tank when turned back on otherwise!