No clean vs. water soluble???
There seems to be this general perception, and understandably so, that no-clean fluxes are "better" than the ones that "need" cleaning. For the majority of hobbyists, this belief seems misguided. Each new flux since the tried and true rosin medium activated (RMA) flux a.k.a. the stickiest stuff known to man is designed to fix some aspects generally at the expense of something else.
Advertisers are wise not to point out the negatives introduced by their new fangled product but you can glean clues into the issues from what they claim is improved. For example, a lot of no-clean fluxes claim excellent wetting ability. If that doesn't make any sense it's probably because wetting wasn't nearly as much of a problem with RMA flux. Wetting in essence is what the flux is suppose to do. It causes the solder to flow better, to ball up like a drop of oil in water. No-clean/no-residue fluxes aren't as good at wetting in general or their wetting doesn't last as long. That doesn't make it bad but it's a trade off. For beginners this is generally a poor trade because shorter wetting periods mean you have to be quicker and more skilled to use the flux. No-clean fluxes also tend to be very difficult to clean.
On the other end of the spectrum are the water soluble fluxes which also tend to be acid based. There are two issue with these. The first most people will encounter is the water part. Water has really high specific heat which means it takes time to boil it off. It will suck heat from your iron immediately. To deal with this you need a high end temperature controlled iron and again more skill. Another major issue with acid fluxes are the fumes. Guess what, they're acidic. Very acidic. The fumes will make you cough immediately. I always wear an acid respirator when using acid fluxes for hand soldering. They also need to be cleaned off which is actually less of a problem for most hobbyist since these fluxes can be cleaned with a little warm water. The benefit over the RMA and no-clean fluxes is that acid fluxes work really well at wetting. i.e. it does it's job as a flux.
So which is better? Which is best?
It depends what you're trying to do. I have all three types of flux on my work bench and I use all three regularly. If you're not sure, the Rosin flux RMA (Kester 186) is a good place to start. I will strongly recommend using a RoHS Lead-free solder. There's no reason to needlessly expose yourself to lead. If you need to be convinced that lead free solder is just as good, write me with what you are having problems with and I'll try to give you some pointers. There's no doubt that SAC305 (3.0% silver, 0.5% Cu, the balance tin) which is what I use behaves differently from 60/40 or 63/37 Sn/Pb but it's not objectively worse, just different.
One area where water based acid fluxes seems better is when used in solder pastes. In the "easy bake" soldering method where you use a stencil to squeegee the solder paste onto a board and then bake the PCB in a toaster oven the negatives of acid fluxes don't really matter. The fumes are less of an issue since you're not hunched over the board breathing it in. The water is also less of issue since there's very little water in the paste compared to bottled fluxes and for most people, controlling the toaster oven is rather tricky so the better wetting and temperature resistance is more important.