Professor! =D The ultimate strategy character, and the most flexible out of any character in the game.
Defensive: A well played Professor is damn near impossible to kill if they're well prepared. They can basically design their own GTB from skills, between Magic rod taking care of single-target spells, and Land Protection against AoE spells. They also have Spell Breaker, which stopped anything with a cast time. It's not too handy in high rates where Instant cast is easily achieved, but it's awesome in lower rates. In terms of offensive attacks, they have Safety Wall, and if they wear an elemental armor, they can endow their opponent to have that element, essentially nullifying damage. When it comes to battling anyone, using the Wall of Fog is amazing to dodge and reduce damage. This is also your biggest defense against Snipers that you can get. Your Energy Coat helps even further cushion any blow you might receive.
Support: Any Professor who plays full support, to me, is wasting too much of their character. D= However, they're pretty useful for it. Their endows save a lot of money and weight limit for your team mates and party members, and it allows you to constantly change it for them as you notice changes that they might be too busy attacking to notice. You can also use floor enhancements, which, while generally ignored, are actually handy. Especially Deluge. Find a good High Wizard, and the two of you will be a pain in everyone's donkey.
Trouble Maker: Your endows can actually act as a trouble makings kill - something that makes it harder for the character to do what it needs to do. If you think someone is about to use a fire endowed strike on one of your team members who as ED, change it to Earth to get rid of the fire. Not to mention Dispell, the single most irritating skill to be used against any character in the game, and the sole reason, it seems, why everyone aims at killing our sexiness the instant they see us. [ Which can be useful, as some people are so desperate to kill us quickly, they don't realize the two or three LKs and Paladins standing around you. xD ] The ability to dispose of all their SP is also full of sex and awesome.
Magic User: Pretty self explanatory. They have the strongest combo attack in the game. [ Spider Web + Stone Curse + Mind Breaker + Double Cast + Fire Bolt = One hell of a hit. However, this really isn't recommended against your fellow magic users, just in case they come out of this alive at take advantage of the super-charged magic power they now have. ] They also have the more typical attacks, and are just a general pain. Most intelligent Profs before to freeze or Spider Web their opponents before attacking, so constantly being frozen or trapped is a real irritation.
Melee User: All right, I have never actually seen anyone use this against other classes, but... Well, there aren't many characters than can do both magic and melee. =D Being a Battle Prof is one of the most entertaining ways to level yourself up.
Marketeer: This might confuse a lot of people. xD Especially on higher populated, semi-lower rated servers, endow scrolls are awesome and in high demand, due to the sheer lack of Professor players.Thus, you can rake in a fair amount of zeny playing on this fact and making yourself a scroll writer.
... Their versatility is awesome. =D Next in line for me would either be High Priests, or Soul Linkers if the server are one of those that allow us to use our magic in PVP, too.