楼主可以考虑用 gag 这个字。它一般的意思是“塞口物；箝制言论”，但另一个比较不常用的意思是“笑话；幽默之处”，正好配合“笑点”所想表达的意思。我个人认为它比 "punch line" 更能表达“笑点”的意思。
笑点在哪？ = What's the gag here?
下面是从外国一个字典网站上复制过来的解释，以供参考：
gag
noun
1. a joke, especially one introduced into a script or an actor's part.
2. any contrived piece of wordplay or horseplay.
verb (used without object)
3. to tell jokes or make amusing remarks.
4. to introduce gags in acting.
5. to play on another's credulity, as by telling false stories.
verb (used with object)
6. to introduce usually comic interpolations into (a script, an actor's part, or the like) (usually followed by 'up').