Short answer there's is none... long answer you can argue.
In simple words it's true that finding a "best" image hosting site that won't conflict with Bitcointalk's proxy can be quite a challenge. As of now, there isn't a definitive answer to this question.
At one time imgur was one of "the best" image hosting site but time has proved it wrong.
After that ImgBB is working fine until today, but you really don't know what will happen tomorrow.
Last
https://talkimg.com/ has appeared very recently and yes it is owned by someone known to the forum has promised to be dedicated to bitcointalk and may hold promise but again there's no guarantee whatsoever that this is the best image hosting service. or there are no guarantees that it's the best solution for Bitcointalk's image hosting needs.
On a lighter note I'm using talkimg and really thank joker_josue for his amazing service but since it's not official "Bitcointalk" hosting so we're relying on his dedication.
So in summary really there's no single best hosting service for bitcointalk until theymos brings bitcointalk's own image hosting service or may be that new software already has a built-in option for image uploads who knows? Until then, it's a matter of trial and error to find the most suitable solution for your needs.
for now as it works fine on bitcointalk.org. The cool part is that you upload image and it returns you BBcode which you just need to past here. So talkimg.com has made image uploading a bit easier on bitcointalk.org. uploading image on bitcointalk.org is not as easy as uploading a file on Gmail. We need to wait until there is some modern way of image uploading introduced here.