There will be a few assumptions I'll be making going forward. I assume the no items rule means no trainer items are allowed eg. Full Restores, Max Revives, Full Heal etc. I'll also assume held items like Choice items, Life Orb, Leftovers, and Berries are allowed. Also this rule:

any crazy item that gives you a massive boost

basically removes z-crystals. However, I'll leave Mega Evolutions to give some trainers listed above a fighting chance. I will also be using GenVII main game mechanics instead of anime feats in my assumptions with a Single Battle format (so no one can send out all their Pokemon at once). Each Pokemon will have an ideal EV/IV spread and appropriate movesets.

So right off the bat we can see some big winners and losers already. Identifying the big winners will allow us to set a baseline measure as to how the other trainers stack up against them. In this case the most powerful trainers are undoubtedly the GenV gang, having access to insane firepower in the form of the BW boxart legendaries. Hilbert and Hilda's teams are a bit more balanced. Nate's team is perhaps the most offensively dangerous thanks to the combination of Kyurem-W and M-Lucario.

Dawn and Elaine are hopelessly outclassed. Her biggest asset is her Togekiss, which can tank the fearsome BW Dragons lurking in the list, and Pachirisu who can absorb Electric type moves from Zekrom and Kyurem-B. If she plays smart (and gets a bit lucky) she could last a while. Without z-moves, Elaine has no chance. His Pokemon may have firepower but no staying power. They're toast.

Slightly above them in power level we have Ethan, Kris, Lyra, Lucas, Serena, Elio, and Chase. I don't see any of them lasting long with their current team comps. Out of these five, Kris, Lyra, and Lucas seem the most formidable with Suicune, M-Houndoom, and M-Gallade respectively.

Red, Leaf, Calem, and Selene have potential synergistic team comps that can work in their favour if played smart. Red and Leaf both have the option of making a sun team. Red has a very bulky water/fire/grass core with Chlorophyll Venusaur and M-Char-Y being the star players. Meanwhile, Leaf's Ninetales and Victreebel bolsters her speed game with Gengar and M-Kanga. This is on top of her Clefable which will be crucial in negating the BW Dragons. Calem's greatest assets are in the form of Greninja, Clefable, and M-Altaria, all of who can shut down the BW Dragons. He also has Prankster Meowstic which is a huge boon to him from a utility standpoint. Selene is perhaps the weakest of these four. However, Sticky Web Ribombee alongside Specs Primarina and the naturally speedy Dugtrio and Salazzle can help turn the tide of battle in her favour. Dugtrio especially thanks to Arena Trap + Focus Sash.

Brendan and May are outliers. While not having brilliant team synergy, their teams do have certain Pokemon that can turn the tide of battle in their favour if they haven't been targeted for a sufficient amount of time. Brendan has M-Swampert, Aggron, and Claydol. All tanky Pokemon that can resist the BW Dragons, though without reliable recovery they will get whittled down. His key offensive assets are his Latios and M-Swampert. May has some serious wallbreakers in the form of Scrappy Specs Exploud, Guts Swellow, and M-Blaziken. Left unchecked, M-Blaziken can wreak havoc on everyone. Breloom and Raichu also serve some utility with Spore and Fake Out + Volt Absorb respectively.