Kalameet is hard for first timers. By the time you Youtube the strategy and have memorized its attack patterns it is actually pretty easy since you don't need super reflexes to roll. The trick to cutting off its tail is to maintain mid~range distance so as to trigger a higher chance for its glide attack. The next thing to note is to roll to its opposite side at its back after it has landed, and that will trigger its tail smash attack 80% of the time.
I honestly think that Seathe's tail was harder to nail. =/
Manus on the other hand...you must learn how to deal with its 3 dark spells and even then the hardest to deal with is its Dark Bead spell. It has fast startup and can't be blocked.
Manus gets my vote, but if you've spent a few hours practicing against them, both are manageable.
I love them all though - especially considering how only few bosses in the original Drk provided the adrenaline rush of fighting Maneaters, Penetrator and Flamelurker. Shame there's nothing quite like Storm King in Drk.