For me its level four. I don't like how you have to be so precise with the aiming.

also level 4, not because of the aiming but because even if you memorize where you need to click that fucking shat out godzilla goes way too fast and to random directions so if you miss a shot its a reset

im not a fan of 3 i find it a lil boring

i mean, all the levels are boring, if you're playing full game all that matters is how fast you beat the boss anyway