Well, I'll mention the bosses I thought were good, and ones I thought were "weak," and what could be improved.
The spider boss in the Deep Roads has been mentioned. I liked it because it was challenging but not in an unfair way--it would summon spiders and web my whole party, but it wasn't a situation where I would have to pray to the RNG gods to survive the web. It just meant I had to be quick about recovering after the web.
The revenants/gaxxkang were all pretty good. They felt tough to kill, and were resistant to some AI abuse like kiting, and had abilities you could react to if you were fast (you can run out of range to avoid Double Strike if you're fast enough, or get behind cover to avoid getting pulled into range).
The Broodmother was pretty good, because of the phased feel of the fight and the different elements to watch out for. I liked the adds and the tentacles, but what I thought was weaker here was the opportunity to get into positions where the Broodmother couldn't damage you. I don't think that should ever be possible in a well-designed boss fight.
Jarvia was a pretty good boss fight. She was dangerous, took a long time to kill, and seemed to switch targets more than other bosses. Her calling for help at low health, and the trapped areas in the room, kept the fight from being non-trivial and made geography significant. What I didn't like about this fight, and many others like it, was the forced conversation/grouping up at the beginning. I know it's so your whole group is there in the cutscene, but I wish there were a way of avoiding the problem.
The Archdemon was a "cool" fight because of the named NPCs appearing and the continuously spawning darkspawn and highly mobile boss. Interesting fight and felt fairly epic. The breath attack seemed pretty weak, though.
The Arcane Horror near the werewolves lair was a lot of fun the first time I fought him. He teleported around, got help from friends, and cast a variety of spells on my party. The second time he was insanely easy--you can attack him with ranged from beyond his casting range, and while he was considering that, my warrior PC and Sten ran down and chained their stuns/knockdowns on him.
Dragons in general should have been great bosses but fell flat in a few ways. It is just way too punishing and flat-out annoying to melee dragons. You're far better off having everyone use a ranged weapon and healing through the fire spits and breaths. Grab was often just a death sentence if I got in melee, since most melee stuns felt too unreliable. Cone of Cold worked best to save the tank, but it's impossible to stay in range as a mage with wing buffet and everything else happening.
The four-faced statue mentioned earlier wasn't a boss, but was still annoying because of my party's behavior. If I didn't put everyone on hold, my party would run to the middle trying to attack the statue and ending up doing nothing if I didn't manually move them away. Even mages and archers did this.
Some general bosses that oculd have been much cooler are the ogres in general. I feel like I should be able to dodge the "Ram" attack, but it's impossible, and can be pretty brutal in some situations. My warrior PC ran into one by himself in the Fade and had to reload numerous times because the Ogre would just ram him over and over again, or pick him up and bite his head off. It's ok to get stunned or knocked over in a boss fight, but they shouldn't be chaining abilities like that so much. I've gotten rammed 3x in a row, and there's absolutely nothing a sword/shield warrior can do to stop it (their stuns take too long to wind up).
Other general issues that don't necessarily apply to bosses that would make encounters more fun/meaningful:
--Lower the range on bows, crossbows, and staves. IMO it's a problem when I can attack from far enough away that the AI doesn't know how to deal with it. I would say if I can shoot them, they should be smart enough to run toward me (and not one at a time!).
--It's been said, but make it so enemies and allies no longer attack forcefielded targets. It's annoying when my allies waste their time and mana/stamina attacking an immune target, and it's comical when the enemies do it.
--Beaf up the hostility system. Taunt is amazingly powerful--with it, everyone attacks me. Without it, I can't get a monsters attention to save someone's life, it seems. Threaten helps only a little. Currently monsters seem too disposed to attack and stick with their initial target. It's somewhat annoying when I can't pull a monster off Wynne when I don't have enough stamina to taunt, and it's just stupid the way enemies will follow you forever once you have aggro.
Maybe enemies that use melee should check every 5-10 seconds for a closer target if their current target is out of range. Running away *should* be viable for a bit, but I feel cheesy when I have a character try to run away just to survive, and end up kiting a boss in a circle for half a minute (or he'll kill me) while everyone else kills him. Either that, or more bosses need an effective method to deal with kiting (switching to ranged attacks, calling help, running faster, something).
--In general, diminishing returns on stuns/immobilize effects, both on the party and on enemies. A shield warrior can very nearly keep a mage knocked down or stunned indefinitely by rotating abilities. You're limited by stamina in that case, but with a little help it's not a problem. Stuns *should* work to lock down a boss or the PC for a short time, but as I mentioned earlier they shouldn't be chainable indefinitely.
--Raise the cooldown on cone of cold. It's a great spell, very useful tactically, and I love it, but that cooldown is just amazingly short in relation to its duration, even with just a single mage.
I know I mentioned a lot of things that aren't really boss encounters, but to me the design of game mechanics plays a large part in how boss fights go and how interesting and challenging they are. I can (and do) just choose not to abuse certain game mechanics/spells, but it still cheapens the experience knowing that I have to hold a hand behind my back to get a challenge. IMO I should be able to play my hardest and use all of my abilities and not feel like I'm cheating.