Flux in the form of pens, pastes or liquids all depend on the flux type used. Where I work, I have found halide based fluxes more than 20 years old still work far better than modern equivalents. They are very corrosive and require thorough cleaning to prevent rapid deterioration of PCB & components. I'm unsure if these are still available, and require ventilation when in use.
I find in general, RMA fluxes give best results with hot air soldering as a certain amount of time needs to elapse while the flux is in its 'activated' temperature range. The slow temperature/time ramp hot air provides is ideal.
Organic based fluxes typically work at room temperatures so they are both effective and require cleaning (a lot are alcohol based and de-min water can be used to post-clean. A lot cheaper and safer than Iso)
Another thing to consider is the source of your components. If your IC's for example have been stored for years without desiccants, oxides will form on the pins requiring a more aggressive flux to get a reliable solder joint. You'll know when you get a bad batch of IC's as they don't wet as easily as they should when soldering.
All fluxes have a shelf life, and should be stored in a refrigerator. I personally don't bother as I'll use a 50ml bottle in a few months.
Flux cored solder is just about the standard these days. They are said to have a shelf life too, though I have a 10yr old 1kg roll that I use day to day and can't say it works any better/worse than a roll a few months old. One thing I would recommend on an old roll of solder is to give it a wipe over to remove the lead oxide that forms on it. It doesn't effect the soldered joint but dirties your tip a lot quicker.