Nice question!
I have always been a lizardmen/seraphon "purist"...
<< Seraphon, or nothing! >> 
But if I imagined GW had never made the Lizardmen range, my answer to your question would be:
-
Chaos demons (in order from favourite to least favourite: Khorne, Tzeentch, Nurgle, Slaneesh)
They are a lot, they differ a lot from each other, they are nasty, disturbing, violent monstruosities.
So yeah, nice stuff.

Since in a fantasy game what I look for is something different from standard humans, Deamons sound like the right choice (after Seraphon, of course

).
-
Death faction
Although I'm not a big fun of thousands of identical skeleton on the battlefield or Death's troups in general, I admit there're some incredible models I force me not to buy.
Mortis Engine, Terrorgheist, Wargheists, Spirit Hosts, Vampire Lord, Wight King, Cairn Wraith (but also lots of things from the "old" Tomb Kings range)... They are all amazing.
P.S. To me, when choosing an army, my choice is based
- 85% on the look of the models
- 5% on the style of gameplay
- 10% on the price of the miniatures of that race
- 0% on the lore of it
What about you?