Hatchbox PLA

It is the number 1 filament sold on Amazon. It offers good quality
at an attractive price.
There are more exotic options out there, but Hatchbox is a good filament for
all your general purpose needs.

Push Plastic.

If you are want to spend a bit more for a higher quality made in America filament, I would recommend Push Plastic. I especially like their bulk reels that can help bring the cost down and reduce the number of times I have to swap filament. Bulk reels really start to make sense if you are running some parts in volume on your machine.