Whitney surprisingly never gave me a ton of trouble, neither in HGSS or back when I played the original GSC games. Clair, however, gave me a significant amount of trouble both times, and surprisingly I also think I had to challenge Chuck more than once in order to beat him. Tate & Liza gave me a lot of trouble consistently in RSE, though, so I will definitely go with them! Another honorable mention goes to Maylene, who crushed me the first time I played Diamond.