I've only had bulk apothecary and whatever @Whiterose0818 uses, which seem equal. I have a gallon from nicotine river that I haven't opened yet. If you read on BA, I believe they mention there are only a couple domestic suppliers of glycerin. That said, if you want guaranteed purity, then Nude Nicotine is where it's at.
I'll bring in @Big_Benny_MI who bought NN nic more recently than me. Mine seems a bit harsh, but I vape 0% 95% of the time. I've vaped <20ml of 4% nic in the last 2 years. But over a gallon each of PG/VG.