I have used some fluxs over time, some liquid, gel, paste, tacky.
For my suprise fake amtech RMA syringes (Gold Yellow color from outside view) works very well for cleaning, since them i don't use anything else but these to cleaning or remove, after i clean the board, IPA works when liquid or hot, if it solidify you need to use flux cleaner. Still i find leave flux in is not a problem with these. Fumes are ok, not bad at all, still you should have a extractor.
for NC flux the best i ever used was the InterFlux IF 8300, that thing works like magic, and is a bit tacky so it hold the component better. A downside is the fume it produce, it easly put you underperformance if you breath that thing.
http://www.interflux.com/en/rework-and-repair I also use NC amtech syringes, they work good but they turn black residue if you keep the iron there for a time. Fumes smells like burn coal.
For liquid flux i use some pens in the past, flux there sucks, so i buy one liquid from ebay that work very well, i can search for it if you interested on liquid flux. But InterFlux have liquid flux and i bet they are very good:
https://www.ebay.com/itm/50ml-INTERFLUX-IF8001-no-clean-LIQUID-solder-flux/331566898405I use paste flux for wires, so i stick the wire there and tin
Overall i only use syringes, the idea of have a can and you need a tool is messy and need more time to apply. syringes are easier.
And why is there so huge difference in price for this another AMTECH RMA-223
One can design look like original, other don't. You still don't know about the inside content!
Mechanic flux is also good, is a real chinesse repair shop, they use quality stuff