That's a real toughie, I'd definitely have to have a tepig/embroar, a ferrothorn, Hydreigon, and victini. Maybe a heatran and feraligator
Teoig is there because it's always been my favorite starter, and always ends up soloing the elite four.
Ferrothorn because it's pre-evolution always stuck out to me
Hydreigon because it's just that cool
Victini brings back good memories of playing pmdgti (pokemon myster dungeons gates to infinity) and its signature V-Wheel
Heatran js one of those legends that every forgets exists
And feraligator because it's full evolution line let's me play through (and beat) pokemon crystal in less than a day
-Nyx