I love makeup brushes, and I have a lot of them. Good makeup brushes made a HUGE difference to the quality of my makeup application. I do rcommend that you invest in good makeup brushes, they are worth the extra price tag. And if you take care of them, they will last you a long time. Check out my brush cleaning tutorial.I recommend that you buy makeup brushes one at a time, instead of all at once. Choose brushes based on what you need. For example, summer is on its way and you got a bronzer, and now you need a good brush to apply it. Don't just buy brushes because someone else loves them, or because they're limited edition, or because you're at the makeup counter, and the makeup artist told you they love that brush. My best brush purchases were researched ahead of time and/or planned way in advance.
The following is a review of face brushes I recommend. For each category, the order of the brushes is my order of preference. I have all of these brushes, and have used them all for at least 1 year - which is the last time I bought a makeup brush. For several reasons, most of the brushes I use daily are MAC brushes. MAC brushes are good quality (you get what you paid for), MAC stands behind their products (I have exchanged a faulty brush once), and there are MAC counters everywhere. Also, I learned a lot about makeup application from youtube gurus and fellow bloggers, and they all use MAC brushes. I found that having the same brushes made it easier to replicate the looks.
Although I think the brushes mentioned below are good quality, and recommend them to you, I realize that they are quite expensive. There are cheaper alternatives out there that are similar in shape and performance. I recommend the Sonia Kashuk line of brushes - good quality at much more affordable prices. If you are vegan and want synthetic alternatives, check out Lumiere Cosmetics.
Regarding Sigma brushes:
I know that Sigma Makeup makes a whole series MAC 'dupes'. I have not tried these brushes, and I have heard mixed reviews, so I am reluctant to recommend Sigma brushes to you.
A note about MAC brush numbering:
MAC brushes are numbered, which I found really confusing when I started. I have not found much pattern in the numbering system, other than face brushes are numbered in the 100s, eye brushes are 200s, and lip brushes are 300s.