That is a good question.
Though a few of them can be inferred by main series stat knowledge.
Of the top of my head, the ones I can think of that could be great are Archeops (basically better offensive stats then ramardos, flying subtype might be liability in some cases?) Hydreigon (dark/dragon pseudo, 'nuff said) Conkeldurr (already theorized to be an improved version of machamp when released if same moveset, higher attack, hp, physical defense, though I do not know how the speed mod will affect it) Hydrocannon primarina, with the current stat system, and if it gets water gun will be above kyogre DPS last I checked, Chandelure has the potential t out Ghost DPS gengar. For short, the bulk of the pokemon that threaten to outclass current ones are gen 5 pokemon (as the gen was essentially a soft reboot to the series, many were made with old ones in mind, but slightly shifted stats, the best compairison being machamp and conkeldurr, same base stat total, very similar distribution, but a few points here and there differ).
Megas I wouldn't worry about the stats of those, as most o the ones that likely will be tier topping are already amazing right now; tyranitar, salamence, gyarados, etc. Only one I can think of that woul dbe potentially stand out are the fighint type megas, as current best fighting types machamp, Haryama and Breloom all do not get megas, currently 'inferior' fighting types Blaziken, Lucario and depending on moveset Gallade all have the potential to take that spot with their mega stats because the whole point of them it to be broken. Or mega mewtwo X, assuming it keeps its same moveset, as its psychic/fighting, and has that high of an attack stat as is...add STAB and more attack, yeah forget what I said about the others.
Legendaries are a separate matter, as they at times get the best moves; Rayquaza, SB mewtwo, AS moltres, Raikou, and others they dont; heatran, regigigas, dialga,
The only pokemon that currently have no threats are Rayquaza, Meteor mash metagross, potentially mamoswine as its only competition is Kyurem, a legendary