Whatever the game is built to accommodate, and staff is capable of handling.
The biggest issue some games run into, IMO, isn't that characters are 'too powerful', but rather that a game is not designed to handle characters of that power level. By and large, most games are made to accommodate low to mid-power characters - by which, I mean characters who may have significant power, but based on the personal or immediate environmental level. A game can handle a mage who can throw fireballs, a werewolf who can tear one monster apart, or a businessman or gang leader who can control a building or small territory just fine.
Most games are not able to handle a mage who can blanket a city in ice, a werewolf pack with a spirit who can tear apart whole swaths of monsters, or even a mayor or lord who can command police forces, armies, and make changes that fundamentally shift economies and social structures.
High power characters need //different// challenges, not just challenges with higher numbers behind them. It's a struggle to accommodate them in the typical 'find thing, beat up thing' plot without making it either trivial for them or impossible for anyone who isn't them. I think a game has to be built to give them an arena that really showcases the privileges and perils of power...and most games aren't. Hell, most GMs aren't experienced in or interested in GMing plots at the upper end of power. That's true in tabletop, too - if you think about it, most tabletop games never get out of that 'heroic' phase, and the acquisition of real power is the end of a campaign.
So, I don't think there's an ideal power range for a game, but I do think a game needs to define the power range it's interested in, and then design itself around that, including not allowing apps below that range, and requiring retirements for characters who exceed it.