Like many it ultimately depends on how they are written but I will say this. I tend to buy into the noobs learning fast and winning than I do pros losing. So for me I always like to see what an author plans to do with an ultra powerful character who is a good guy. They have to find ways to basically write them out of certain conflicts so that they don't just solo everything. I particularly like what you can do with causing a sense of dread in readers when a really powerful ally is defeated or killed. The sense of despair can be pretty intense if their power and how much they are relied upon is really built up. One instance that has always stood out to me was early on in the Naruto manga when they killed off the 3rd Hokage. I am also a sucker for the history and power levels of stronger characters. Like if an author tells you character A fought character B to a standstill 10 years ago but character A had an injury at the time I am always like "well what would have happened if he didn't have it? How much would it have changed?" and things like that. Can't get enough of it. On the flipside though, I hate when they never explore it and you are left with all of those questions even when the series ends.