to be honest, these days, I mainly use ebay for whatever they have that's really cheap.
The quality of even the cheapest pla has gone up so much in the last few years, that price point is my main criteria.

That said I do have favourite filaments.
Flashforge semi-translucent red pla is an awesome filament, good price and just great to print with and tough as old boots !
I'm also a huge fan of sienoc flexible pla. It's every bit as good as polymakers polyflex - but half the price and has an almost pearlescent finish.
For basics like black and white filaments - it is usually whatever is cheapest at the time.
Been down as low as £9 with delivery (p&p is very important when you're dealing with items that mostly weigh over 1kg)

Amazon prime is you friend. I have it for my business anyway.
That said you can also get free postage from ebay - if you do, make sure it's shipping from YOUR country. Or the scumbags at the inland revenue will charge you a lot of money to import it !