With all of the new gems and items what is the best t16 build? I have seen some of big Daddy’s builds but I’m not sure which one is the fastest more efficient.
Will probably just make Monk and DH even faster 
Seriously though, every class can make a super fast build. Also depends on if you’re targeting DB’s, Keys or just farming for mats.
It’s going to be hard to beat set based builds for this reason in terms of all round efficiency.
Okay, I’m going to assume that you mean a WD farming build seeing as you posted here.
As Jay mentioned, there are faster classes, and it does depend what you are doing.
If you are doing bounties, HT Pets + Chicken (Manajuma’s set) hands down to cover those distances.
Your absolute fastest will be Barrage w/Garry or Darts–either of which, kitted out for speed (Hex/Stalker, Krelm’s belt, Severance rune for SW, Ring of Larceny, etc.).
My favourite for non-bounty farming is LoN/D Petwalker. It is maybe ~<1minute slower than the above and is SUPER chill to play. You almost do nothing with the Locust Swarm variant, so you can watch videos, etc. while farming. Check containers, pick up loot and stay away from bad stuff, that’s it.
is the HT chicken build in S18 strong enough for T16 with 4 people group?
Chicken Doc works fine for this but it doesn’t kill as quickly as other builds like Darts, Spirit Barrage or even Jade. Pets don’t always target what you want them to and damage might be lacking a little bit in a 4 man depending on your gear. It is still a solid build for t-16 rifts and one of my favorites due the relaxed easy playstyle.
Which is faster darts or spirit barrage?
I would say Darts is a tiny bit faster. With Spirit Barrage, one phantasm will kill pretty much anything but it doesn’t explode right away. I always end up running back a little to pick up the deaths breath. Darts does damage immediately.
What about LOD firewalker? How does that compare?
For Season 18, I did T16 solo in under 5 minutes using a Helltouth pet walker build. The new Flavor of Time amulet effect (“pylon effects last twice as long”) was a huge help.
As I understand it, the more progress you make in a rift, the more likely a pylon will spawn. I was getting through the rift so fast, a new pylon was spawning like every 90 seconds. Thanks to the Fierce Loyalty passive and Krelm’s Buff Belt, I was able to get around the screen and catch the pylons as I did my run.
I had between 580 and 650 paragon points at the time.