I have foundation brushes, sponges, and puffs..but 95% of the time I use my fingers to apply/blend foundation..
I personally think it's the very best way SO LONG AS your fingers are clean. I always wash my hands prior to putting on my foundation..And I carry sanitizer with me in my makeup bag.
Using your fingers adds warmth to your face and foundation to help it meld into your face the way brushes/sponges just can't. In fact, I've read most m/u artists like to use their fingers (but sometimes cannot, due to health standards). If it's YOUR face, then why not? Like I said, just wash them hands!
I have noted I have a MUCH harder time applying foundation with my $50 brush than I do with my little fingers! It's a LOT more time consuming, and tends to make it look heavy if I don't spend twenty minutes blending!