Over and over
In one world, it was the Storm Guardian who dies first. It was pretty hard to get to him, since he rarely left the side of the Tenth Vongola Boss. But still worth sacrificing all that canon meat (and really, that's all Byakuran's pawns ever were to him, even the strong or interesting ones) – without the Leader of the Guardians, with their Leader robbed of the Right Hand Man he came to rely on greatly over the years, Vongola's resistance became chaotic, driven only by revenge and not common sense. Nice every once in a while, Byakuran noticed.
In another, it was the swordfighter. Taken out while away from the base and still trying not to kill his attackers – he didn't know yet, had never heard of the Spells or their Leader, never suspected a real threat to the great power that the Vongola possessed – and paying first with his arm, then with his life. The Vongolas vengeance spree for their Rain Guardian was terrifying, wiping out whole areas, suspecting every ally – it was good fun, Byakuran desided.
Once, memorably, he started with Mukuro by turning off the life-sustaining machines in his tube, out of anger after having a hard time dealing with the Mist Guardian in a different world. He hadn't thought how strong the bond between the illusionist and his vessel girl was. Mukuro only gained power abandoning his own, restrained and decaying body and taking over hers completely, warned the Vongola and was, once again, being a real pain.
Byakuran made it a point to kill the Mist's vessel first next time.
Getting a grip of the Cloud Guardian was a hard task, as even the Vongola Leader himself had a very vague knowledge of Hibari's whereabouts. And on top of that it always took a couple of Byakuran's best fighters to take him down. On the upside, it took Vongola a while to start missing their Cloud Guardian and in that time Byakuran managed quite a lot – and Hibari was unable to turn up at the most unlikely time and proceed to slow down Byakuran's elimination of the other Vongola.
Starting with the Sun proved to be a bad idea. Not that Byakuran particularly cared, but while it darkened the whole Sky, it also left the Leader with his two hands, his most loyal followers, to avenge their friend – with controlled anger and carefully planned attacks, aware of him and his strength – a major headache to deal with. It simply wasn't worth it in the end, Byakuran concluded. It didn't stop him from following this path from time to time.
Killing off the Lightning first was unsatisfying and counterproductive. He still was the weakest of the Guardians, even if he probably held the biggest potential, and he was guarded well by the others. To complete his list Byakuran once managed to start the end of the Vongola with the elimination of the child (13 at that point). He had to face the Vongola Base group afterwards complete with the Vongola Decimo, Storm, Sun and Rain, which, while being actually challenging for once, also proved to be really tiresome.
Of course, starting with the top seemed to be foolproof.
It was always a fine challenge to go directly for the most influential man in the Underground, Vongola Decimo.
It was hard getting past the Guardians of Storm and Rain, the Tenth's nearly-shadows, ever harder getting past the damned Vongola Intuition, and of course, the fight itself was either over after a second or could go on for days. But it was all worth it – after the Sky cracked, the Guardians lost their spirit. And no matter how the Storm tried to coordinate their last defenses, no matter how much righteous anger the Sun and the Rain or how much bloodthirsty vengefulness the Cloud and the Mist put inside their attacks – it was over as soon as their Leader stopped breathing, and both parties knew that.
This was foolproof. In every world, over and over again.
Except for the one time when it wasn't.