Ehh I've tried a few fluxes here and there, for soldering DFN SMT, and I find it rather hit and miss.
On the one hand, ebay has tons of stuff, usually in large easy-to-apply bottles, labelled 'no clean flux'. Whilst this can work, it often floods, and instantly cools the iron, leaving lots of steam, and by the time its hot enough to melt the solder, the flux has gone. If applied with a cloth, or a cotton bud - does the job just fine. (I still clean off the residue though, as it generally wipes clean with isopropyl alcohol). Almost always for ROHS, lead-free solder temps. The stuff used for leaded solder is much more forgiving - I can swipe the pins of a TQFP in one pass leaving no solder bridges - all whilst its flooded.
On the other hand... there is expensive brand names from RS, Farnell, (and rapid in the UK) available in pastes, and pens, both of which are excellent and leave beautiful shiny joints even with cheap lead-free solder. They are however expensive.
Also, as most solder wires these days come with flux cores, usually resin, sometimes no-clean, these can leave residues themselves, another reason to clean. I've only used water soluble fluxes once, never again. Sure you can clean it with water, but if any residue remains under a SMT cap in an oscillator, or something, it can cause all sorts of troubles. Even some no-clean fluxes have caused this problem for me
I settled on buying one of those pen applicators. Expensive, but I really use very small amounts.... plus I can refill the pen with cheapo flux if needs be. Cheap in the long run, does the job. Like the OP I'm also looking for effective flux for lead-free soldering (mostly SMT parts) that isn't ridiculously priced
BT
Edit: just noticed in that link the 'Kester 186' - thats the stuff I've used time and time again. The nib gets fluffy and old, but it works like a charm.