So here are my thoughts:
Charlyne Yi as Frisk and Chara


Seth Rogen as Sans
Paul Rudd as Papyrus and Grillby


Kristen Wiig as Toriel Dreemurr

Andrea Libman as Alphys

Eileen Montgomery as Undyne

Hugh Laurie as Mettaton and EX

JK Simmons as Asgore Dreemurr

Grey Griffin as Bratty and Catty

Steve Buscemi as Burgerpants


Cameron Diaz as Snowdin Shopkeeper
Tara Strong as Temmie and Monster Kid

Helena Bonham Carter as Muffet
Iain Armitage as Asriel Dreemur


Brad Swaile as Asriel Dreemurr (God of Hyperdeath)
And that's it for now...