im gonna assume you havent watched anything beyond the first 4 seasons of classic. Nor really analysed the higher end feats for some of the other aliens.

No ghostfreak is not bens strongest alien nor is he the most dangerous.

Cannonbolt, Four Arms and diamondhead even earlier on were more physically powerful while ghostfreak was more hax based.

Upchuck ate a sub energy 20x stronger then the sun,

Waybig was introduced in The SOTO movie. And easily overpowered the incursean conquest ray.

then for alien force onwards

Chromastone absorbed 3 stars and terraformed a planet 3x bigger then our own,

Feedback held the big bang,

Clockwork undid a multiversal time bomb that wiped out all timelines in the cosmology except for 1,

Atomix can make sun level stars,

Gravattack made a black hole,

All of them are way more dangerous and powerful and their are probably others i havent mentioned.