Major Work, Masterpiece, Blockbuster
noun
The big make! What is your big make? Bigger than anything else. It is your major work! 大作 means a large-scale, ambitious, or significant work. Depending on the context, it can refer to anything from an artistically accomplished masterpiece to a big-budget blockbuster movie.
たいさく
This is a jukugo word, which usually means on'yomi readings from the kanji. Since 大 has two on'yomi readings, here's a mnemonic to help you remember which one to use: Did you know that there's a monumental work about a tie sack (たいさく)? It's a masterpiece about a magic sack that produces tie after tie after tie after tie... A bit like The Magic Porridge Pot, but with a sack and ties instead.
「サイコ」はヒッチコックの大作だ。
"Psycho" is Hitchcock's magnum opus.
「スターウォーズ エピソードI」は１９９９年のアメリカ一の大作でした。
"Star Wars: Episode I" was the most epic American film of 1999.
もう自分では大作のアイディアは考えられない。
I can't come up with any ideas for a masterpiece anymore.