Sweet, my favourite Death Monster right now, in Gold, is either Undead Rexx or Grim Reaper:
Grim Reaper really just shuts down a lot of sustain comps in one single attack. It's nice to shut down Sea Monster, Cerberus, Haunted Spirit, Flesh Golem, (Kron if he's not with Llama!) etc. Plus it completely invalidates a lot of player's Wood Nymphs, Crustacean Kings, and Divine Healers too. The few times I see a matchup with Sacred Unicorn, he also helps greatly.
Then there's Undead Rexx, who has a massive health pool and is just tanky enough to potentially completely ruin somebody else's team with a trample. And in the trample modifier he's just so fun.
Also, RIP Gigas