Longer version. Doing higher GRs only increase the xp, blood shards, and chance of getting a higher number of legendaries. The quality of the legendaries themselves isn't effected. In theory, assuming you have done a solo 70, you could get a primal ancient from a gr 10. The reason they seem to have better gear is because they are probably playing more, and doing those higher rifts faster gets you more shards etc, which gives you more chances at legendaries and in turn better odds you will get that ancient/primal you want.
There's multiple builds that can clear really high solo GR for next patch with all the buffs. Many can do 110+ with decent para. As for the "highest" it's hard to know. Cause the ptr has undergone many iterations this time round, with multiple ptr leader board wipes and such. A strong contender will probs still be bloodlance necro which can do like 118s right? It's hard to fully compare where everything lines up because not everyone has the time to properly push on ptr, or maybe they don't even want to show the build cause they have some secret strat. The highest I have seen is a Korean streamer attempting a 127 with dmo wiz. He was close to downing the rift with almost enough time to kill the rg. Keep in mind he has 5.5kpara, 9mil sheet dmg, and he's Korean. With his para and skillset he'd probs push levels beyond the rest of us mere mortals with any build he focused on
Edit: I misread and thought you wrote "best build" singular, not plural haha. Oh well I'll let my reply stand as is.