Who is supposed to be the highest-power character in the Dragon Ball series? Is it Lord Beerus, Gogeta SS4, or somebody else?
The most powerful character in the Dragon Ball universe, currently, is Whis, Beerus' assistant.
This is stated in an interview with Akira Toriyama, the creator of Dragon Ball:
I suppose if Beerus’ strength is a 10, [Super Saiyan] God would be right about 6. [...] Incidentally, I guess Whis would be about a 15.
In an interview with Akira Toriyama, he states that Whis is the most powerful character.
To be honest, in my opinion, Goku is the strongest due to the fact that his power has no limits. He has seven super saiyan forms, and currently, it is SUPER SAIYAN GOD 2 (-sweatdrops- Seriously?). He also has plot shields, and the trope DeathIsCheap.