I find the more challenging boss fights to be the best ones. So here's my list.
The boss fights are actually the most disappointing thing about TW2. Most of them are too easy and predictable once you figure out the opponent's pattern/moves. It's also why I've advocated for more difficult/less predictable boss fights on the higher difficulties.
In order of difficulty:
The Operator - Most difficult battle.
Letho (1st time) - You're not that powerful and he's more powerful than you. This isn't too easy of a fight. I have a hard time beating this one without signs.
Dragon - By the end of that game you're extremely powerful. Once you figure out the technique, this fight is quite easy.
Letho (2nd time) - Not too hard this time, however, more difficult than the fights below.
Draug - Aard, hit hit hit. Aard, hit hit hit. etc. This guy is simple. I should try him without signs, I guess that's a better gauge of how easy/difficult it is.
Kayran - I can beat this one with no signs except Yrden and no poison potion. It's not difficult at all. The only difficult part of this fight is running up that platform at the end. Getting hit by a rock on Dark Mode pretty much kills you.
I guess all of those fights were much more difficult the first time through. I remember reloading the Kayran fight so many times because I couldn't figure out what to do (too much dialogue skipping I guess) and Letho's first fight took me forever. The dragon was also ridiculous the first time.