The fourth question on Jeff's list is "Who is the mightiest wizard in the land?" The actual personality of the mightiest wizard is a matter for you to decide for your own setting (and keep in mind that an ambitious player of a Magic User will probably have the title of "mightiest wizard in the land" in his sights, so keep both the current title-holder's attitude about that and how your PC Magic User can achieve the title in mind), but there remains the question of what makes this wizard the mightiest in the land. Here are some ways that a wizard can be the most powerful wizard in your campaign.
Answer 1: This wizard is just the highest-level wizard around. She had the most raw talent and the best work ethic and is also pretty old; she'll be legendary not only for her power but also for the adventures she's been on and the feats and services for kingdoms she's accomplished. In a lot of ways, she'll be like Yoda. (To replace her, you'll need lots of hard work over lots of time.
Answer 2: This wizard possesses spells that no other wizard has, or at least more spells than any other wizard. This can be because your setting has different spells available to different wizards, based on who they apprentice with or where they study, or it can be because this wizard has researched and invented a bunch of new spells. (To replace this wizard, find a way, fair or not, to obtain or copy his spell book, or to amass even more spells than he has.
Answer 3: This wizard knows a powerful, non-standard type of magic that gives him access to a privileged spell list. In the Delos setting, Dragons have their own type of Dragon Magic with their own powerful spells. Dragons are loathe to teach them to non-Dragons and will do so only for the very best of reasons; some spells they will completely refuse to teach to non-Dragons. Somehow, through extraordinary service or guile, this wizard has obtained at least some Dragon spells and learned to cast Dragon magic; or perhaps this wizard is a Dragon! Another way to do this is to make other arcane classes, like illusionists or Arduin's Runeweavers, into prestige classes that this mightiest of wizards has taken, making their spell-lists available to him. (To replace this wizard, learn the secrets to this non-standard magic, either through apprenticeship, replicating this wizard's adventures or trickery.
Answer 4: This wizard knows many rituals that few wizards have even heard of and even fewer are willing to practice. The quickest way to do this is to let Magic Users perform Carcosa rituals on top of regular spells. (This kind of wizard won't be disposed to sharing or teaching, so stealing the spell book or killing the wizard are probably your best bets for replacing this wizard.
Answer 5: This wizard specializes in a weird sub-section of magic that is very powerful and esoteric. A good example would be the many prismatic walls in Arduin that take a long time to learn, take a good amount of time to erect and are very hard to dispel. (To replace this wizard, study under him, or go get your own weird sub-section of magic to specialize in; stealing his book will probably not result in much, as it won't make sense at all to a standard wizard.
Obviously, all of these can be combined with each other to make the "mightiest wizard in all the land" even more powerful.