So at this point I've beaten every boss at least twice and some of the other bosses like Radahn and Malenia four times each. So this is my original boss ranking impression so far.
15. Fire Giant - I don't think he's really all that terrible, but at this point, I think he's the worst boss for several reasons. The first being that he's kind of boring. If everything goes your way, it still ends up being a long boring fight. I fought him both on horse back and off and honestly, I couldn't tell you which the developers intended. Either way he rolled around wayyyy too much and dragged the fight out. Twice he got stuck up on the ledge and made it impossible for me to get close to him without getting bodied. And one time he rolled on top of me, hitting me twice, immediately killing me. I think he has some AI problems, area problems, and just general boss problems. I like the idea of fighting him like Midir but his openings weren't big enough and he mostly just spammed magic and rolled away.
14. Radahn - I don't see why people love this fight so much. With summons, he a joke, on horseback he's a slog, and off horse back he's a total camera beast. I've never found myself enjoying this boss and I don't see how they could fix him at this point.
13. Regal Ancestor Spirit - Really just a boring, easy boss. I don't know what more so say about this boss. Nothing caught my attention fighting it both times nor did I ever feel challenged. I felt mildly annoyed during the healing and teleporting but that's about it.
12. Fortissax - Arguably the most impressive looking fight with probably the sloppiest moveset. I couldn't really tell you what was happening during this fight but I got through it both times without dying. I can't say the boss was bad at all, just nothing notably good.
11. Astel - A cool boss in theory that ended up being kind of annoying. It teleports away too often and has a really annoying grab attack. Add in some pretty crappy gravity magic and you have a fairly mediocre boss. I did like a good portion of its moveset, but those portions were ruined by other really annoying attacks.
10. Renalla - First phase speaks for itself. Second phase, just kinda feels boring. No complex dodging, just running. I can't say anything was really annoying, but nothing stood out either. Run up to her, attack, repeat, win. Kind of a let down.
9. Malenia - To be fair, I think Malenia has potential to be one of the best bosses fromsoft has ever made. However, at this point, I feel like she's a cheater more than any other boss. I think she has a fantastic moveset and I don't mind the complexity of waterfowl. My three biggest gripes are health steal, animation cancels, and waterfowl charge up. Give her a bigger health bar, remove health steal (or lessen it), add a quarter to a half second of wind up to waterfowl, and remove her ability to cancel her getting up animation into a kick or waterfowl and I think we'd have an easy S tier.
8. Radagon & Elden Beast - A sure-fire s-tier combined with an utter piece of garbage. Radagon is probably my favorite boss in this game and possibly my favorite fromsoft has produced. Radagon has been the highlight of my run each playthrough. Elden beast on the other hand is steaming garbage. Not all the time though. He's really just RNG if RNG were a boss with some garbage moves. Totally brings down the radagon fight.
7. Goddrick - Now we're getting into the bosses I have very little trouble with. I think Goddrick would be an S-tier boss if he was a little faster, had fewer openings, and had greater moveset variety. I love his design, I love his moveset, I loveeee his transformation. However, the fight feels over a little too soon and a little too easy.
6. Morgott - Him and himself (Margit) are just quality boss fights with the same problem as Goddrick; the fight is over too soon. I still don't feel like I don't fully understand his moveset, but I look forward to learning it better in the future. So far, the fight has just ended too fast to really feel like I needed to get good at it. Same with Margit. Maybe they just need a little bit more health.
5. Placidusax - Another solid fight, that I found very fun. I love his moveset and his design. I love the teleporting and reappearing part. He has very cool attacks that don't feel annoying. Maybe my only problem with him is he's a little too slow. I'd love to fight his lightning claws more face to face like midir, but his booty is really the safest place to be for now.
4. Rykard - This boss went from a D tier to A tier the second time I fought him. The first time, he trapped me against a wall and had me stuck in his stupid fire for awhile. I wasn't pleased by that. I also didn't understand how the explosive skulls worked so I was constantly frustrated. The second time, I had a lot of fun. I really like his moveset that they fixed in 1.04 and I thought fighting both the god-devouring serpent and Rykard were really fun. I also might be biased because I absolutely love the artistic design of this fight. (Shut up, I'm not moving Radahn up.
3. Horah Loux - Just a genuinely fun, difficult, and rewarding fight. I loved learning both phases of this fight. Great moveset, great opportunities for jumps and strafes, and it really rewards you for knowing his moveset inside out. The two problems with this fight are 1. his hitboxes are a little ridiculous and 2. I fought him on xbox one and the frame drops during this fight are for some reason so much worse than any other fight.
2. Mohg - A great contender for #1 in my mind. I loved the change in pacing in this fight compared to other Elden Ring bosses, I felt much more like a DS3 fight. I also liked racing against the clock to see it I could beat the boss before phase two. I never did, but I always enjoyed the fight. The only thing I would change is how long the fire stayed on the ground.
1. Maliketh - Easily my second favorite boss (after radagon of course). The second phase is absolutely amazing with how you can bait his attacks with good positioning. His moveset was very difficult but very rewarding to master, in fact, my second playthrough, I beat him no-damage first-try. It was a great feeling after he wiped the floor with me 30+ times my first playthrough. Again, I think the only thing keeping this fight from S-tier is Beast Clergyman. I like how jumping is actually really good against him, but I feel like his projectile spam adds too much time to the fight. It was also quite a bit harder than most fights with colossal weapons.
Unfortunately, I don't think any bosses are a solid S tier. Some are very close but as for now, that spot is yet to be claimed. Hopefully some updates in the future fix this.