I'm just going to rank the arch-demons, but go ahead and rank any you want. 
 
1- False King - Easily my favorite boss at the end of my favorite level.  Everything about this fight is difficult and intense.  you can't run or use cheap tactics on this one (unless you do the poison cloud thing or arrows thing which takes hours.  He will destroy you if you use a shield and heavy armor, so its about lowering defenses and rolling.  lots of rolling. 
 
2- Old Monk- I wa so surprised when I placed my blue eye stone in 3-1 and ended up getting summoned by the Old Monk.  I love the PVP aspect of this game, and making a boss another player is just brilliant.  I loved fighting and playing as this boss. 

3- Maiden Astraea - This fight was just weird, even from the opening cutscene.  I see a priestess sitting in a pool of blood with a heavily armored bodyguard.  I personally did not fight Garl Vinland, I did the cheap way and went right and killed her with arrows.  Then I stupidly jumped down from the archery spot and got destoyed by plague babies.  The entire scene is just creepy.  A priestess, surrounded by blood and creepy skinless babies who will give you the plague, and if you kill her bodyguard she commits suicide. 

4- Storm Ruler - I hate manta rays.  I hate them so much.  This would have easily been my least favorite fight if i didn't know about the storm ruler sword.  The sword is the only reason I liked this fight.  The joy of slaughtering manta rays three at a time was great.  The boss itself was annoying to say the least, especially when every one of his 20 or so spike things he shot hit me.  I never actually died against this boss, and now i have a great area to grind for souls 
 
5- Dragon God - this boss was intimidating, and without the use of the wiki i was completely lost.  I grabbed the Dragon Bone Smasher first (sweet sword btw) and ran off to the right.  the boss immediately trapped me and roasted me.  It took a few tries before i realized how to avoid getting killed.  Overall just a tense and not-fun boss fight
 

1. Maiden Astraea - I feel as though she has one of the most interesting stories behind her and she has some good dialogue.
2. King Allant - Although not really a proper boss fight he has some very interesting things to say.
3. Storm King - Just because of the cool sword ability you get to use while fighting him.
4. Fool's Idol - I also love the story behind this boss and how she's surrounded by her blinded followers.
5. False King - Basically the same reasons as you said.
   

In terms of gameplay
1. Maneater:      Just enjoy him dunno why
2. Flamelurker: speed and aggression
3.False King: forces your A game on you. No mistakes allowed
 
In terms of story: 
3. Astraea: so heartbreaking and such a nice creepy horror vibe
2. Adjudicator (*): Awesome. You ate a chicken nugget sometime? Guess what, your staying as a skeleton :)
1. Tower Knight/Penetrator: Where maiden Astraea followed her charity into the netherregions they followd their sense of chivalry. And they still make for damn fine knights!!
 
(*) just started another playthrough with my now lvl49 gal. Died to adjudicator twice. I know embarrasing. Both times I got stuck in a corner by some treasure and had to time my rolls but couldnt really reach his wound. I don't remember that happening. But then again I'm super rusty and NG is hard at her level :)

My personal top 5 favorites eh.....
 
1: Maiden Astrea - She has the best story and lore of all the bosses save maybe Allant himself, her fight makes you think about the game world and your character a little too.  It can be approached and beaten in multiple ways and she even gets the best theme music.  Just a great set piece that really brings out what Demon's Souls does right.
2: Tower Knight - I don't think anyone can deny he has potentially the biggest on screen presence.  There are cheap ways of beating him sure but it is overall a very well set up fight, plenty intimidating your first few times, and fits so well with the story and the level itself.  One of the iconic fights of DS.
3: Fools Idol - A strange choice I know but I really like the theme of the Tower of Latria and how each of the bosses fit into it story wise.  She has a ton of atmosphere to spare, can be a really creepy opponent, and fights in a pretty interesting way as well and feels very different from other bosses in the game.
4: False King - The real last boss of the game the "put up or shut up" boss.  Lots of lore goes into the character, he has tons of buildup for the fight, very central to the plot, and a challenging fight besides.  He also has alot of non cheap/glitchy ways of fighting him too and even though he has some seriously heinous things he can do feels very balanced and all his attacks fit perfectly with the character.  He is a grade A boss any game should be happy to have.
 5: Dragon God - It is the dragon god, nuff said really.  It isn't a "fight" at all but it works really neat, is done well, and has loads of buildup for it.  I don't know what it is about the guy really but it just feels right to me.  Also he is another one of those iconic DS bosses and it feels good to get revenge for that falcon punch at the end of the tutorial, assuming you got to see it.