When you're painting with acrylics or watermixable oils you're pretty much stuck with using synthetic brushes since hog-bristle brushes get floppy really soon when they get into contact with water frequently.
Until a few months ago I primarily used W&N Artisan brushes since they were stiffer than most synthetic brushes but then I came across Rosemary and co, a small mailorder company in the UK. All their brushes are handmade and you can request a free catalog which displays the brushes life-size so you won't have to guess about the size of the brush.
The best brush for acrylics and watermixable oilpaint IMHO is the Ivory line of brushes. Stiff but springy, it holds quite a bit of paint and it retains its shape a lot better than any other brush I've worked with. I like the long-flats and the filberts.
If you like a brush that is just a bit softer they also make a really nice synthetic Mongoose, named Eclipse.
The top 2 are Ivory brushes, the bottom one is an eclipse.
If you're looking for a great brush for regular oil paint you should try the Mongoose series 279, it's like the Langnickel brushes but without losing the hair :)
Apparently Jeremy Lipking, Morgan Weistling and Richard Schmid use these brushes as well . If they are good enough for them..
Update: 11 August 2010: Since I've switched to a softer oil paint I currently use primarily the Eclipse brushes combined with the Series 279.