I discovered NC.com about 1 1/2 years ago and started CG shortly after that. I do not use ONE single product from before that time -- I've changed absolutely everything. So my longest-running favorite is Jessicurl Aloeba
(about a year). I'd put Jessicurl HCC
in that category except that I've almost stopped needing to use it since I've switched to products that rinse out easily.
I have enough Boots to last me quite a while, and I suspect that may wind up being a long-term keeper too.