When I did a post about which brushes for the eyes I consider as most important to have some of you asked me to do one for face brushes as well so here you are :) I have to admit that it´s much harder to pick out the most important face brushes than eye brushes since there are so many factors to consider (what you like, what kind of makeup you use etc), but I am going to try to give you an idea of which brushes I think are better than others.
1. Foundation
It depends a lot on the kind of foundation you are using. Some foundations, like Nars Sheer Glow, I actually like to apply with my fingers while others all have their own “winning” brush. Basically I tend to prefer to apply thicker foundation with high coverage with my fingers as they tend to dry quite quickly. I find that if I use my fingers I can distribute it over the face more quickly, and then I might go over it with a small blending brush for eyes (Mac 217) to get more cover where needed.
I like to apply foundations with medium to high coverage that don´t dry too quickly with a Mac 131/Sigma F15 wich is a “Flat, tapered brush that contains a combination of short and long fibres” – Mac.

This brush is actually ment to be for blushes and highlights, but I find that it works brilliantly for foundations as well. I use it with small sweeps and strokes to blend it in and it gives a very pretty result.

When using a more light weight and liquidy foundation I like to use a MAc 187/ Sigma F50 (or a Mac 188/ Sigma F55 which is the same only cheaper and smaller) because the foundation won´t dry into the brush as quickly because of its consistency and it´s easier to blend it in with a round flat top brush than with a tapered brush like the Mac131/ Sigma F15. The result is an even and natural soft finish. I use it by firstly stippling the foundation around the face and then swirl it in. I do however advice you to swirl just a little bit more than you think is enough because the result is so much nicer and you´re sure to not have any streaking anywhere.

2. Mineral foundation
If you´re using a mineral foundation the kabuki Mac 182/ Sigma F45 is obviously one way to go. Sigma does great kabuki brushes and I like the flat top for more coverage and the rounded for normal coverage.

The flat top requires some work though. Sigma also recently launced three High Definition brushes ( F84, F80 and F82) which many people like. Personally I prefer the normal kabukis because I find it easier to get a nice and even result then with the HD brushes which eat a lot of product and are a little bit too dense to give that nice soft finish. Some people also like to use these HD brushes for liquid foundation, but this doesn´t work for me at all.

The flat normal foundation brushes (Mac 190/ Sigma F60) are only used to apply face masks in my house. I think they are useless for foundation as it takes a really long time to apply and gives a streaky result.

3. Powder
If you have an oily skin and need to use powder all over your face I´d go for the Mac 134/ Sigma F20 because it´s quite big and therefor reaches over a big area with fewer swirls. Not only does this save you time, but it makes it easier to get the right amount of powder as well.

If you have rather dry cheeks or just like to put a little bit of powder in your t-zone I´d go for the Mac 129/ Sigma F10 as this brush is more or less the same shape only smaller. Since it´s so much smaller it´s obviously easier to apply more precisely in smaller areas like between your eyes without messing up your eye makeup. This brush is also nice to use for both blush and bronzers.

4. Bronzers/ Sun powder
If you are using the bronzers to get a sunkissed look (not for contouring purposes) I´d go with the Mac 138/ Sigma F25. This is a quite big brush but it´s tapered on the tip which makes it easy to get to smaller areas as well. Because it´s tapered and round it actually does all the blending work for you because of the shorter hair on the sides. Brilliant brush! It´s also great for use on the body since it´s so big.

5. Blushes
For liquid blushes like Benefit Benetint you are better off using your fingers as the liquid dries very quickly. If you´re not quick enough with blending it in you risk getting an uneven and spotty finish.
For cream blushes I actually like to use my fingers to apply the blush and then a brush like Mac 188/ Sigma F55 or Mac 131/ Sigma F15 to blend out the edges. Remember to apply just little by little instead of too much at once. It´s much more difficult to blend out a cream blush than a powder blush.
For powder blush I find my self reaching for the Mac 168/ Sigma F40, a medium sized angled brush. I like it because it gives an even finish and it helps with blending since the longer side blends it out for you as you apply. If you´re in a rush and get a harsh line it´s easy to blend out harsh edges with the longer side instead of having to use another brush. This brush also works great for bronzing, contouring and highlighting. Just like with the 217 blending brush I´ve actually lost track of how many I have of this as I use it for lots of different products.

If you have a very pigmented blush like Mac Fleur Power or a blush with lots of glitters you may find it easier to apply the blush with a brush like Mac 188/ Sigma F55 as it doesn´t pick up as much product on the brush compared to other brushes.
6. Contouring
For contouring (using a matte powder in 1-3 shades darker than your skin) I prefer to use Mac 129/ Sigma F10 (medium sized blush brush) which is great for temples and Mac 165/ Sigma F35 (small round tapered blush) which is amazing for cheek bones. The Mac 165/ Sigma F35 is actually the same as the egg shaped Mac 138/ Sigma F25 only smaller. Remember to go over and blend out edges with a bigger clean/powder blush.

If you´d like to contour your nose as well I advice you to go for a big blending brush for the eyes like the 222/224. Be very careful when contouring your nose, you don´t want obvious dirt on your nose ;)
If you´re wondering about what brush you should go for when using concealer I wrote a couple of words about it in this post about brushes for eyes. :)
There are some slight differences between the brushes from Mac and Sigma. I do prefer the brushes from Mac as they are better quality, but the brushes from Sigma are a third of the price Mac is asking for them and a lot better than brushes from other brands in the same price range so unless you´re a makeup artist you are more than ok with brushes from Sigma.
Sigma offers a package where you can choose which brushes you want to use to build your own kit. This is a genious way to go for getting the brushes that you need for the cheapest price possible. The price per brush ranges between $14 and $19 whilst Mac charges £22 to £36 on maccosmetics.co.uk (a little bit cheaper in US).
I hope this was helpful and please do ask in the comment form if there is anything I can help you with :)