The word "most eruptive" implies something that is prone to sudden outbursts of energy or emotion. Antonyms for this phrase could include calm, peaceful, placid, serene or tranquil. These words suggest a sense of stillness, stability, and balance, which is the opposite of the energetic, explosive nature of eruptions. When seeking to describe something that is not prone to sudden outbursts, we might use terms such as controlled, measured, stable or subdued. These antonyms help to balance out the extreme qualities of "most eruptive," representing a kind of emotional equilibrium that can be soothing and reassuring.