So honestly this is a pretty hard one because I like a lot of the sponsors. But in the interest of this post I will limit it to two.
First, bulk reef supply because I honesty don’t know where I would be in the hobby without there videos and information they share. Honestly the only reason I buy from them is because I feel like I need to show them support for everything the do for the reefing community.
2nd, is more R2R specific. No company has been more responsive to me and been helpful on this site then @ecotechmarine. There customer service starting on here and ending on the phone with actual reps is second to none
I have two as well. @Bulk Reef Supply because they provided me with a ton of information as I've been learning (new to the hobby in March). Their videos are unparalleled in depth of knowledge yet are presented so the beginner can get a grasp of the concepts.
The second is @LiveAquaria as I've ordered all of my livestock from them. I've had two guarantee claims that were settled quickly and to my satisfaction!
July 23 entry: It is extremely difficult to pick 1 amazing sponsor on this site. I have to say it's a tie between 3 though. @Bulk Reef Supply because let's be honest they are the go to for information, customer service and just giving back to the reef community! @TopShelfAquatics because they are my LFS and have killer live sales! Last but not least @CoralVue because their customer service and products are top notch!!!
July 23rd entry: favorite sponsor @Bulk Reef Supply
I think their youtube videos series are very informative, cover most of the topics you will have to learn on reefing and the experiments also allow you to do your own conclusions for your setup/tank features.