This is tough. Alright, in not particular order:
1. Solo crow (the one with Shisui's Koto). I could use this to project Koto on the moon and command the world to obey me.
2. Rinneihawk. Hawkglance, hawkblitz, hawk tensei... Choose how you'd like to be solo'd.
3. Sakura. Imagine hearing the summoning "poof" sound and as the smoke clears you hear "SHANAROOOOOOOOOOOO!" and a fist of death flying towards you.