Archaon was definitely AoS, as mentioned he came with the Varanguard and the Gaunt Summoner.
My favourite centrepiece model is probably the Eidolon of Mathlann, I really like Katakros, Kroak, and maybe controversially Lauka Vai too. The Kruleboyz vulture and croc thing are great too. I also love the Warsong Revenant. Outside of large models, I think my favourite ‘unit’ is probably the Dread Pageant from underworlds.
But the best kit, in terms of customisation, aesthetics, paintability, character, everything in combination, is probably the Rockgut Troggoths. They really did knock that one out of the park and I don’t think it’s been matched since.
Edit: if we include models that came out in WHFB but are still around, I have a soft spot for the Treelord. Really cool aesthetically and a really clever build that offers a lot of customisation.