I like super bosses in games, I enjoyed Malenia fight same reason I enjoyed Weiss the Immaculate in FFVIIR and Sword Saint in Sekiro.
They are epic and absolute thrill to fight them.
Edit: Also "equal level" only works in PVP not in PVE when AI is not all that smart…that kind makes you are not really in "equal level".
Bosses are not suppose to be equal ground, they should much stronger have more dangerous attacks which in my opinion makes the fight more fun.
That's why I used quotation mark on "equal level". The fight is design to feel like a battle between 2 hunters. Even if one is a boss. (And the battle is much more interesting that PvP xD)
I like impressive big boss. In Elden Ring, I loved Godfrey and Placidusax (my favorite boss of the game, EPICNESS x10000) are good examples.
Sword Saint is also another example of good very challenging fight. It's always fair and reward the player for learning timing, pattern... while pushing you to your limits.
(Haven't played FFVIIR yet)
I use a lot the word "fair" because it's an important aspect of a good game/boss for me. Since Dark Souls, FromSoft has master the art of "challenging but fair" in my opinion. But it's not the case of Malenia in my opinion. Her waterfall attack is really infuriating for me. I question also a bit her staggers which looks inconsistent to me and cost me countless death. Her regen is fine in my opinion.
To be honest, several times in Elden Ring I had the feeling they "failed" a bit on this "challenging but fair" aspect. I hope it's not a consequence of the constant talk about how their games are difficult and they feel they need to push further the limits. We'll see their future works. I know the incoming DLC will be probably be a step further in difficulty like it always has been in their DLC (still looking forward to it).