Well... Dumbledore was credited with the defeat of Grinwald, but that doesn't really say too much since Grinwald's involvment and abilities were never revealed. Still, Dumbledore was also credited with a lot of other things, and when you're as old as him you do tend to know a few things (although senility does kick in quite viciously around then).
As to the arguement of HP magic vs other systems, HP magic is weakest simply because of now it is. (Note: those who want to just get a basic understanding of what I mean, skip to the bottom paragraph, those who want to go in detail or just dont have anything better to do, disregard this note)
In HP-verse, magic is mostly for peace time. Most spells are for personal comfort, sports, and entertainment, with a bit of combat spells for normal policing and animal control. The most important things to those wizards are preserving secrecy with a predominant isolationist and "not broken don't fix" view, so a great deal of combat spells would be forgotten/lost simply because they are not relavant, and new combat spells would not be developed for the same reason. The exception that is Voldemort/other Dark Lords is just that, exceptions that come every few decades and are usually over in a few years, so after the defeat of the dark lord it's just business as usual again.
Now, for just about all other universes where (casted) magic is available, such as Slayers, D&D, Negima, etc, it is used widely as a method of survival. For the Slayers and D&D-like universes, you have rampaging monsters, demons, bandit gangs, hostile tribes of orcs, trolls, drow, etc, etc running across the countryside. If you are an adventurer mage, you would most likely be packing some serious heat before you ever step 10 miles from your village/town/city, that is if you have any sort of survival instinct or common sense at all. By neccesity, to deal with the "wild and untamed" world, combat spells are widespread and REALLY nasty to be on the recieving end of. Even in Negima, although it is more similar to the HP-verse, mages actively go around the world and quest as opposed to the HP-verse's isolationist policies.
To sum up my above chunk of text, HP-verse doesn't really NEED a huge amount of combat spells for their regular lives/policing/animal control/secrecy, so powerful spells are forgotten/dont get developed. Other magical universes, however, NEED nasty spells that'll stop a rampaging demon or an army of trolls parked outside your city gates, so powerful combat spells are kept alive and taught to just about any graduate of the nearest mage academy, while new ways to hurt/kill/maim/inconvience the guys that want to eat you are constantly being researched.