As mentionned in the guide, you can indeed do all the endgame content, included superbosses (the build is actually quite good for those). With a bit of practice, you can do crucible runs consistently in about 5"-5"30 (I myself am not a good pilote in crucible, and I managed to do a 4"50 run). As for SR, you can easily do 75-76 runs, and if you are interested in pushing higher SR, well Duskdeep86 pushed a 99 with a slight variation of this build. Keep in mind all of this was done in hardcore, I think you can achieve more impressive results with a more agressive approach.
“the best” is a very relative concept in GrimDawn imo . It really depends on which area of endgame content you want your build to shine. It is also a matter of personal taste^^.
I suggest you to try your best with items you like, and to define precisely what you want for endgame content.
For example this build [1.1.9.5] My Deep SR Pet Conjurer - SR125 solo, Callagadra, Lokarr, Ravager has absurd results, I mean, SR 125, it really is something, but what the matter if you don’t care about SR ?
You might prefer a build like this [1.1.9.1] Chaos Lost Souls Cabalist - CR 3:49 if you want a glass canon able to destroy crucible in no time (btw this one is a Lost Soul cabalist and is imo very fun to play, I myself played a very close variant of this build). But what if you don’t care about crucible ?
You see, there’s not a definitive answer, you can, probably, find a pet build that is objectively the best at doing, say, crucible (and even then, rng, mutators, pilot skills, make an “objective” approach difficult). But I doubt you’ll ever found the very best pet build at everything xD.