I'm fairly new to gel fluxes myself, I currently have MG Chemicals 8341 and Chipquik's SMD291 (sample that came with low MP solder). Both work well. Of these two, I am leaning towards the MG Chemicals because it seems to be non-conductive and does a nicer job, but it takes a little more time with the heat than the chipquik stuff on oxidized surfaces.
For non-rosin liquid fluxes, I have Kester 951 and Chipquik CQ4LF. Both of them are considered no-clean (AFAIK). The kester stuff works great on clean surfaces and new components and leaves almost no visible residue and does not show conductivity once dry (my go-to liquid stuff right now), but it is not active enough to touch an oxidized surface and it evaporates fairly quick under heating. The CQ4LF is more active and I save that for more oxidized surfaces that the kester can't do. The oldest dirtiest copper pcb in my junk bin can take solder as is with the CQ4LF liquid flux, but I don't find myself using it that often. Only downside is a small amount of visible white residue and it's fumes are not very pleasant when soldering.
Not saying these are the best options, I've barely tried any, I just know these work ok for me. Collect a few options, get to know them, you might find you use them all for different tasks.
Once I collect a few more fluxes, I want to test how they compare, but on both clean and dirty surfaces and checking the conductivity of the residues afterwards, both when dry and after subjected to humidity.
