That jaw-drop moment when someone walks into a scene and you’ve literally lost your train of thought — that’s what I picture for a strong, caught-off-guard synonym. If I had to pick single words that smack of real, visceral surprise, I’d reach for 'gobsmacked', 'dumbfounded', 'flabbergasted', or 'thunderstruck'. Those carry weight; they aren’t polite surprises, they’re the kind of reaction where your brain pauses and your mouth might not cooperate.
In practice I choose based on tone. For dramatic or literary moments I like 'dumbfounded' or 'staggered' because they read well on the page and suggest stunned immobility. For casual speech with friends, 'bowled over' or 'blindsided' feels natural and vivid — 'blindsided' also implies someone sprung the thing on you unexpectedly, not just surprised you. If I want a punchy single-syllable hit, 'stunned' still works hard.
Small tip from my editing days: match the verb’s intensity to the scene. 'Taken aback' is subtle and polite; 'shell-shocked' or 'left reeling' are heavier and often imply lasting impact. I tend to throw 'gobsmacked' into conversations when something is so weird or delightful it scrambles me — and honestly, it never fails to get a laugh.