Notable rental Pokemon for prime cup:
Starmie can do damage and it can cover pretty much every single type.
Charizard, Rhydon, Gengar, Articuno, Zapdos, and Omastar is a great team, I owned everyone and that Mew guy was no more.
If you let a kabutops or something set up swords dance, you will get owned.
Gengar/Haunter/Ghastly is annoying if it can get the opponent to sleep.
In the cup where there is levels 50-55 Jolteon and Starmie make a good core as rentals. Starmie is just a beast overall.
Jynx can send Pokemon to sleep.
If you have a mewtwo handy, you can own pretty much anyone.
Dragon rage can destroy you in the pika cup or petit cup.