this really depends, are you talking reality or fantasy. in reality we don't really keep track of 'powerful witches and warlocks' we rely more on knowledge to determine these things. in terms of spell casting, hard to say, no ones spells work 100% of the time, different factors can effect each spell. high magick can be done by anyone who can charge and focus, it just carries greater risk than low magick, so it takes practice.
if you mean fantasy, i've always been partial to Merlin, but the sisters from Practical Magic brought someone back from the dead, and of course there's Gandolf.
You have to keep in mind that before the last century or less, witches and wizards hid their identities and their practice for their own, and their family's safety, because they were hunted down and killed. You can find who the most famous witch or wizard is, but we can never know who the most powerful was.
That would depend entirely upon the definitions your using, and the age of the definition. I was just going by the one most seem to associate the word "Warlock" with; a man who practices witchcraft. :-P