Barbarian The barbarian is best hand to hand fighter in the diablo2 universe. He is extremely good at the beginning of the game and still great if you raise him right, at the end of the game (hell). He is good for most opponents in duels. But not against long range fighters so much. (i.e.-sorc, zon, possibly a necro) But a good Strategy is to crank whirlwind and also crank the mastery your weapon uses. Whirlwind and Warcries are best for duels, with the barb's strength you usually only have to get the opponent stuck in one whirlwind to finish him. =)
Amazon The amazon is an amazing character. She is defintely the best if you use a bow and guided arrow in a duel, or even multiple shot is great. She is harder to use sometimes if you dont have the right equipment for her later on in the game, but if you trade and magic find, you will be fine. I highly recommend this character for anyone.
Necromancer As you can tell by the name, he summons dead people =). He is pretty much the worst character in duels, but pretty good for the game though. He uses summons, and spells to weaken and defeat the enemy. He can raise an army of revived monsters which is extremely useful for the game. This isn't the character you want to raise and expect to be powerful in duels, if you have never raised a necro yourself.
Sorceress The Sorc is my favorite character pretty much. She uses a wide variety of Fire,Ice,Lighting Spells to make her progress through the game. She is one of the stronger people for duels, if not the strongest if you know how to use her. She even has a bit of an advantage over a zon with her teleport, thunder storm, and Frozen Orb and possibly Fire Hydra. Try to stick to one type of spell at the beginning of the game, but later on when ur best spell (example-orb) is maxed out, you should learn more spells in other categories. She is also great for questing, but at beginning it can be hard just using some ice spells until you get to a lvl which you can use blizzard at.
PalladinThe palladin is a lot like the barb in some ways. He has fairly good strength, with his aura's on he is very powerful (such as might). He is very usefull to have in your part, with his strong and wide variety of Aura's. But personaly i like using other characters than the palladin. But he is a great character, especially with Fist of the Heavens.
DruidThe druid is a pretty cool character. He can shape shift and use spirits and even use spells. I think he is a cross between a palladin, a Necromancer, and a Barbarian. Which all give him his weakness's and strengt's. I really like to crank shape shift werewolf form. I also, like to have a spirit to help me when in duels or questing with him. He is not that strong while in normal form, but when in werewolf/werebear form, he has a major advantage over characters in different categories.
AssasinThe assasin is a strong warrior. Skilled in Martial arts, and the Art of Assasintry. She uses traps and marial arts to take out her opponents. She also can use claws, and also if you use claws and your an assasin crank claw mastery. She is fairly good in duels, with her speed technique. Also if you get out on the battlefield before your opponent, try and find a quilrat to kill with tigerstrike to get 1 charge to hit your opponent with, it helps. Assasin is fairly good throughout the whole game, and i recommend her for everyone.