There are anime that are better at being what the creators did not really mean for it to be. This is a list of such anime.
Anime often mixes genres. Horror will have bursts of comedy, and drama will include romance, but they are not meant to be more pronounced than the primary genre of that anime. Yet sometimes those secondary genres end up quite memorable and interesting to watch. This is a list of anime that excels at a secondary genre or simply does a scene from another genre quite well.
Steins;Gate
Steins;Gate is science fiction; it has a mad scientist and time travel, and it is very clearly not a romantic anime. Yet a lot of people would claim that the line of Kurisu and Okabe is executed very well, with the characters sharing a lot of chemistry. Steins; Gate is not a romance anime, but it excelled at romance.
One Punch Man
One Punch Man (Wanpanman) has long been considered an excellent comedy about a superhero so overpowered, he grows bored. It is not technically meant to be a comedy, though; it is an action anime, but it is so funny that it can be marked as excelling at the comedy genre.
Love, Chunibyo & Other Delusions!
Love, Chunibyo & Other Delusions! (Chūnibyō demo Koi ga Shitai!) is probably best described as comedy with romantic elements. What you might not expect from this story about teenagers going to school and having strange delusions, is action that is fairly good. It is just a little unexpected, though not unwelcome.
Miss Kobayashi's Dragon Maid
Miss Kobayashi's Dragon Maid (Kobayashi-san Chi no Meidoragon) is a comedy about a dragon maid. Once while drunk Miss Kobayashi helped a dragon, and now she has to deal with all sorts of magical interferences, all of which are very funny. However, there are also action sequences, which are great. In other words, this is another comedy that is good at action.
March Comes In Like a Lion
March Comes In Like a Lion (Sangatsu no Lion) is mostly drama, with some comedy and romance and slice-of-life thrown together. It is already muti-genre, but it also has a wonderfully animated action scene. While that definitely does not make March Comes In Like a Lion an action anime, it shows that it can do action quite successfully.
As you can see, most anime on this list are not simple one-genre titles. They are complex, and it is for the best. There is no need to limit an anime to one specific genre when you can use multiple; just make sure that you are doing every one of them well.