I voted for Gazrow, Lady, and Firelord.
I guess I'll start my explanation with the two D3 bosses that I voted for. I think I absolutely loved these two bosses because of the balancing issues that both presented.
Lady was fairly difficult to tank with a non actual tank class, but still required a squire class to taunt away the mark of vampire. So it presented the interesting dichotomy of needing either two full on tank classes, or a decently enough geared healer and DPS class. Both strategies presented their own unique challenges and due to the fact that we were level and gear capped, it kept the fight interesting for just about every group.
Firelord was interesting for similar balancing reasons. Sometimes having MORE people in this fight made it more difficult than fewer people. More DPS was good, but more ashes and more confusion could lead to easier wipes, this was interesting. Secondarily he pretty much required a second, totally unique set of gear to fight effectively. Having access to fight him but not being able to succeed because not having the proper gear set up was also fun to me. I think ultimately it came down to how difficult the content was versus how powerful our characters are though. Having world bosses as the hardest content for so long has really made the content feel too easy. When D3 was top tier, it was impossible to be over leveled and over geared for it, but with foothills and whitepines, we've pretty much been over geared and over leveled the entire time.
Gazrow got my last vote because hes pretty much the only boss in foothills and whitepines that punishes improper aggro management. His Enrage mechanic really forces an actual tank to need to tank him. Its the one boss where your armor value actually has somewhat of a significant impact. The attack speed of all of the other bosses makes our character's armor value negligible. It doesn't matter if its taking 300 or 240 damage (so a difference of 60 armor) we'll need to be healer after 2 attacks regardless of armor value. At Gazrow, because of his attack speed, we don't have time to get heals off between attacks 1 and 3, whereas on all of the other bosses, with a decent healer, its fairly easy to get heals off between attacks 1 and 3.