CREATE TABLE parties (party_email VARCHAR, party_id VARCHAR)
CREATE TABLE party_forms (form_id VARCHAR)

Find the emails of parties with the most popular party form.
SELECT t1.party_email FROM parties AS t1 JOIN party_forms AS t2 ON t1.party_id = t2.party_id WHERE t2.form_id = (SELECT form_id FROM party_forms GROUP BY form_id ORDER BY COUNT(*) DESC LIMIT 1)
SELECT "t1"."party_email" FROM "parties" AS "t1" JOIN "party_forms" AS "t2" ON "t1"."party_id" = "t2"."party_id" AND "t2"."form_id" = (SELECT "form_id" FROM "party_forms" GROUP BY "form_id" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1)