If you want challenging bosses, do not use D2 as a base.
Diablo II:
Andariel - Spam blessed hammer. Drink antidote if lacking poision res before fight.
Duriel - Spam blessed hammer. His attacks are usless on you due to insane block / defense.
Radament - Use same strat as Andariel. If minions are annoying, then leave area after killing them to remove corpses.
Diablo - Spam blessed hammer. Teleport arround if lacking res / block otherwise just tank and only teleport to save merc.
Nihthalak - Teleport ontop of him and spam blessed hammer as he will die long before any corpses form for him to blow.
Ancients - The only real challenging non uber boss. Just spam blessed hammer until they are low on health in which case teleport away to avoid nuke on death. Do not let yourself be a punching bag and remember to spam pots and hammers.
Shenk the Overseer - Spam blessed hammer. Honestly not worth mentioning.
Blood Raven - Spam blessed hammer. Honestly not worth mentioning.
Baal - Spam blessed hammer. Retreat if he clones or if low on health to town or corner to regenerate and let any effects time expire.
High Counsil of Zakarum - Spam blessed hammer. Hydras can generally be ignored but to prevent excess damage constantly teleport in and out killing one or two each time (most safe way).
Pindleskin - This guy is tough. Spam blessed hammer if you can from a distance as his charge ignores block and is filled with a huge dose of pain when chained. Use merc to take the hits while 1-2 hammers kill.
Lord de Seis - Spam blessed hammer. I forget which boss this is as I thought he was a bady.
The Smith / Hephasto the Armorer / Isual - Spam blessed hammer. Retreat if getting overwhelmed to regenerate and then repeat.
Witch Doctor [insert unpronounceable name here] - Spam blessed hammer. If low on fire res just go out of his range and get the hammers to hit him for a distance.
Coldworm the Burrower - Spam blessed hammer. Use antidote if low on poision res otherwise he dies in 2-3 hits with you taking no damage.
As you can see, D2 hardly has any varience in boss killing, and you can often easilly replace "Spam blessed hammer." to "Spam 1337 soc move + infinity".
A propper boss fight should need some thinking, some physical game stats and some time. Bosses should take atleast 2 minutes to kill and you should be forced to move in some way or another. They should also rely on physical power in the case of RPGs so that they are easier the stronger you are. Finally they should not just be constant action, there should be breaks letting you regenerate, move or prepair.
Example of boss. A forgoton one in the centre of the room. Around the edges are runes of healing (instant use) and mana but in limated supply. Boss starts off being unkillable and attacks you with a non homming projectile attack a few times. The boss then summons 3 tenticals that move after you trying to hurt you and you must try leading them onto certain areas of the arena making them stuck or kill them. If you make them stuck then you will have a long period to attack the boss during which it is uncapable of attacking. If you killed them the duation is shorter. The cycle repeats but at 66% HP it spawns more tenticals and attacks with more projectiles. At 33% HP it goes nuts and the tenticals move faster but last shorter, also the fireballs move really fast and can only be doged via a huge radius circle. Once killed you get some reward and a full reheal / recharge.
As you can guess this is very legend of zeldery but in a more simplified form. The ideas you must use are very simlar and in the end most of the damage should be avoidable.
Poor boss design on the other hand. . .
Boss starts off hitable and is only killable that way. Boss spams high damage moves that can not be humanly evaded and kill you in a few hits meaning you have to chew pots. Boss then moves on to spam instant death moves requiring some form of medic to constantly revive you or else you will lose. Boss has insanly large numbers of HP / armor so that the battle lasts for 10 minutes with the same repetition of mass pot use / rezing. No way to have a pause to recopurate / prepair for another attack on the boss.
This boss design was seen in FFF (Final Fantasy Forever) with the one ghost boss. Overall these bosses are highly annoying and often were seen in the final fantasy series where by turbased combat was all it had. Although some of them were ok in final fantasy, they do not convert well to WC3 unless properly ballenced and should certainly not be combined with elements of a good boss battle as they will only annoy and irritate people further and you must remember that FF allowed people thinking time before actions, in real time WC3 battles you do not have that.
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