Here's the short answer: I don't know!
I think I've found a brush I love, but then I don't.
What I have figured out is that for the style of painting I like to paint, synthetic seems to work best for me. I have a lot of brushes. Too many brushes. Brushes everywhere... but here's the thing: I'm cheap, cheap, cheap when it comes to buying a brush. I've no problem blowing a wad on a tube of paint, but when it comes to a brush? Nope, not gone do it. (Unless they're magic, but we'll talk about that in a minute...) And perhaps I refuse to spend a lot because I also know that I am a brush killer. I abuse them, poor things. After about three months of use, they go to the 'special' pile. In this case, special means a jar of a thousand brushes that never get used but I can't get rid of them because that is sacrilegious. No, I don't have a hoarding problem, why?
Here are the brushes I am currently using... subject to change at any moment...
Loew-Cornell series 795 Rounds, numbers 1 and 2
MozArt Supplies - size 00
These three are my detail brushes. They're cheap. They work. I found the yellow Loew-Cornell brushes at my local art supply store in the watercolor section and bought them eons ago for students... one day I grabbed one and a new relationship was created. They're firm, yet hold enough water to get the job done.
They've been through the ringer with me now and I recently set out to replace them - of course, the art supply store no longer carries them. Of course. A google search didn't get me much closer to finding the right sizes - but finally I found this site that sells them in a five pack, and only set me back $10 bucks! And whatd'ya know, they're not even watercolor brushes! They're for fabric painters. Oh well. Whatever.
The MozArt supplies brush came as a set and was sent to me in exchange for my super power: Instagram posting. I don't use any of the other brushes. They're too firm for me, except when it comes to the teeniest, tiny lines this baby creates. Teeny, tiny brushes don't last long, that's been my experience. So I go cheap because I know I'm going to have to replace them.
For a long time, I used these brushes exclusively: ProArte Prolene Plus
They're great workhorses, and from the picture you can see, they get a lot of use. They hold up fairly well - especially for an abusive brush wielder. I still use them - mostly the mid sized range (4-6) for larger areas and washes.
Remember how I said I don't spend a lot when it comes to brushes? Yea, well, that all went out the window when I found these. My knees got weak. My wallet fell out. All notions of frugality vanished into thin air. There was nothing I could do, I caved. I spent.
And here's the thing, I didn't care if they were great brushes for painting - I just really, really wanted a hand-carved brush set. They're beautiful. A work of art - and yes, they also look really cool on Instagram. Go ahead, judge me. I can take it.
So here's how you can get on the 'I spent way too much on these brushes, but now I look cool on the Instagram' train: I found them on Etsy, created by Jazper Stardust. Don't freak out when you see how much they cost. Okay, freak out - I'm right there with you. I can't believe I spent that much either. But also, no regrets. I'd totally do it again.
As it turns out, they are excellent brushes to paint with. The tips are Escoda 1212 Reserva Tajmyr Kolinsky Sable. They handle beautifully - hold a ton of water and have a nice tip. I like that they have a nice, long neck - it keeps the water off the wood as much as possible.
So yes, they're expensive. But they truly are a work of art and craftsmanship. Seriously, can you carve the tiny handle of a paint brush? Me either. In fact, I couldn't carve my way out of a paper bag - so I paid someone who can. I'm happy I supported a fellow artisan. I've also tried some of Jazpers watercolors - they are excellent.