I use the following the most:
For flavorings -
Wizard Labs - IMO they have the largest variety of brands (ie. TFA, CAP, LOR, etc.), the best variety of sizes, really good prices, and fast very carefully packaged shipping.
OSDIY - Mainly use it for their One Stop Flavors and One Stop Blends, that I really love. I'll add a few other flavorings from TFA, CAP, etc. to my order also if needed. They're very fast on shipping, have super friendly customer service, and I enjoy supporting a small family operated business. They also have an on-going coupon code for a discount - ECF or JFK both work.
For bottles -
GlassBottleOutlet.com - For all my amber glass Boston round bottles and Polycone caps for nic base freezer storage. Checked and compared their prices against other top recommended places...and they always beat them in total price (items + S/H).
DropperBottles.com - As mentioned above for plastic eliquid bottles with a permanent discount.
For nic base -
Wizard Labs, Nude Nicotine, and RTS (they price match, too) have been my favorites and are the ones I have repeatedly bought from.
For syringes & blunt needle tips -
Tractor Supply (syringes 3mL and bigger)
FT for all sizes of syringes, including low priced glass ones and for blunt needle tips that are really inexpensive
For PG & VG -
I always buy it only from Bulk Apothecary. Both are always totally clear, highest quality, carefully packed for shipping, and they have frequent discount codes. Just be sure to sign up for their newsletter. Oh...their prices when including S/H, are usually the lowest, too!