I personally find that the options of the poll don't really make sense, since it's not so black and white between final bosses and optional bosses. As Lei Shen was announced to be the real last boss that shouldn't require further discussion. Sinestra, however, felt like part of the progression, besides even if she wasn't, I think Cho'gall is the least suitable out of the three last bosses to be the final boss of the tier. I consider Nefarian to be the final boss of the normal mode.
Yogg for Ulduar.
There's a contradiction in what you're saying: you consider the last boss that is killed to be the final boss of the progression. For your information, Al'akir was the last heroic boss to die in t14, and Yogg+0 was killed over a month after Algalon.