I'd definitely say Pelipper. Everyone is scared of Pelipper because of its 4x weakness to electric type moves, but if you have, i dont know, a swampert it gets much easier. Pelipper has amazing HP and works great if you can avoid electric types
And i taught surf to pelipper so I could keep better moves on Swampert.
ps. if you use swampert use ice beam!!!or blizzard!
Whitney was never really ever a problem for me in all of my silver runs. The last time I played I knocked out all of whitney's team using my HM Slave furret. its was only like lvl 16 or so.