The party is great! Who organised it?
'Ottima' = OT-ti-ma, stress on first syllable. 'Organizzata' = or-ga-niz-ZA-ta.
Use this as a genuine opener at any party or gathering. Expressing enthusiasm for the event creates positive energy and gives you a natural reason to ask about the host, which opens many conversation threads.
The superlative 'ottima' (excellent) is the irregular feminine superlative of 'buono'. 'Chi l'ha organizzata?' — the pronoun 'l'' refers to 'la festa' (feminine), so it becomes 'l'ha organizzata'. Note the agreement of the past participle with the direct object.
Che bella atmosfera!
What a lovely atmosphere!
Comment on the mood and vibe rather than logistics
È sempre una garanzia quando organizza lui/lei.
It is always guaranteed when he/she organises.
Complimenting the host if you know them
Come mai non ero mai venuto/a qui prima?
How come I had never been here before?
Marvelling at the discovery of the venue
Italian house parties ('feste in casa') are typically very well organised with abundant food and drink. The host ('padrone/padrona di casa') takes enormous pride in the quality of the aperitivo spread. Complimenting the food and organisation is always the right move and will immediately endear you to the host.