When buying solder, stick with a name brand (i.e., Kester). I have some solder I purchased from Banggood that is utter crap. A short section will work ok but the next inch won't even melt at normal soldering temperatures.
Personally, I like 63/37* with a rosin core. Other types of flux, like "water clean" are highly active and will attack the copper traces over time unless every bit is removed after soldering. I have found the smoke from non-rosin fluxes to be highly irritating to my eyes and throat.
* Percentage of tin and lead. 63/37 is the eutectic point of a tin/lead alloy. This means it transitions from liquid to solid instantly with no plastic phase, reducing the chance for cold solder joints.