Really any quality rosin based or no-clean flux will work well.
But Kester 186-18 (lower solids version), standard 186 (RMA, but borders on RA), and 1544 (RA) are my favorites.
Unfortunately, the smallest bottle Kester sells is 1gal.
So Kester's disposable pens are the only viable size they offer for hobbyists. Another member has resorted to using pliers to remove the cap containing the felt tip and pour it into a brush bottle (like those used for finger nail polish). Plenty of other types of dispensers you can try as well, such as a refillable brush pen (i.e. Bonkote BON-102; my favorite, so there's some bias here
). Upside is you can be sure you get genuine product (assumes bought from a distributor), and the dispensing method of your choice.
The downside is the disposable pens are expensive (cost per fluid volume).
Another method that can get you the above advantages at a lower cost, is to do a group buy and split up a gallon.
There are also eBay sellers that break down gallon containers into small bottles, but there's a greater risk of getting a counterfeit/cheap substitution (I made a trial purchase of 186 from a store called
Dickie's Garage Sale, and it seems genuine <smell & performance>).
Beyond this, you'd need to change to a different brand that sells liquid flux in small bottles suitable for hobbyists. For the US market,
MG Chemicals 835 is an excellent quality RA flux that works very well (they offer 4.2oz/125ml & 1L sizes), and is far more cost effective than the pens (i.e. 4.2oz size is ~$9). FWIW, All-Spec is really good for supplies and tools, and they have excellent pricing and reasonable shipping (usually the lowest price on solder, such as Kester 44, so you might want to bookmark their site).
Another useful company to keep in mind would be
CML Supply (they also have an
eBay store), as they break down Kester RF741 tacky gel flux and EP256 solder paste (Sn63/Pb37) into hobbyist friendly sized syringes.
They also produce their own disposable pens using Kester flux, and IIRC, they're a bit cheaper if you want to get it this way instead.