Rascal Does Not Dream of Bunny Girl Senpai is a good choice

best romance animes with an assertive male protagonist

Since the question used “animes” (though I’ve heard plural of anime is “anime”), looks like it’s fine to list multiple shows😈 In no particular order:

• Cherry Magic! - Adachi can be assertive sometimes; Kurosawa is pretty assertive.
• Rascal Does Not Dream of Bunny Girl Senpai - Azusagawa is simply great throughout.
• Skip and Loafer - Not very romantic just yet and can’t remember it well at the moment but I believe Shima is assertive.
• The 100 Girlfriends Who Really, Really, Really, Really, REALLY Love You - Rentarou is pretty assertive.

In fact, my top favourite romance anime tend to have an assertive male protagonist. I can’t stand wimpy, indecisive, weak-willed male protagonists.

The best one that comes to mind for me is:
Ouran High School Host Club