If fit, it should be Thiago, but that's a massive if.
It seems to me that we will continue this box midfield with Trent in that inverted midfield role.
That left double pivot's role is two fold - protect the CBs and help progress the ball in the first phase of play. Jones has no business playing that role. He doesn't have the tactical or positional sense to protect the CBs, and while his technical ability is not bad at all, the first-phase midfielder needs to have a particular skillset - receiving the ball on the turn and progressing it or recycling it. That's not Jones' game at all. Hell, Klopp has spent 3 years trying Jones as one of the 8s in the 4-3-3, and Jones never fully convinced. It took a drastic formation change for Jones to even show that he had a future at the club. Playing as one of two 8s in the box midfield is what Jones has shown he's best at, and I wouldn't be putting him anywhere else at the moment.
Thiago is made of biscuits, but he has vast experience in playing as a deep pivot, and he's excellent in the first phase. He can't offer defensive protection anymore, but I guess Lavia (or whoever else we buy) will take that position long term. Anyway, if he's fit, I'd pick him.
MacAllister can play at 6, but that'd be a waste given that we've spent all of preseason preparing with Szoboszlai and MacAllister as the two advanced 8s.
Fwiw, if we sign Lavia over the next few days, I'd actually throw him in straight.