To be sure, Smalltalk/Seaside isn’t the only fantastic option available. I’ve heard great things about Clojure/ClojureScript and Elixir/Phoenix. It comes down to a matter of individual taste. Ultimately, you have to try them out and see which one suits you best.
Naturally, as a Smalltalk evangelist, I will push for Smalltalk/Pharo/Seaside. ;-)