Here's a real pain in the butt if there ever was one. Aquis floats freely along Oil Ocean Zone's various platforms, shooting projectiles from its snout. While not particularly dangerous per se, this seahorse has a trick up his non-existent sleeve. Most badniks can easily be bypassed by running forward, however, Aquis follows Sonic throughout the level, even up to entirely different floors. Couple that with a traumatic experience from my childhood with one of these and you've got a real crippling badnik.
Beginning the top half of this list is the most annoying bot of all: Mantis. Hiding in the grass of Marble Garden Zone, Mantis jumps straight up. Mostly appearing in speed-based areas, he usually jumps over Sonic harmlessly (making him particularly hard to destroy.) However, occassionally he will not clear our hedgehog hero, stealthily sapping Sonic's rings. He's surprisingly evasive when you turn around to kill him out of spite. Mantis really makes the blood boil.
Burrobot (left) appears en masse in the Labyrinth Zone as a surprise enemy. He jumps out of the floor, leaping at Sonic, then continues on foot--er...tread. Grounder (right, the logical follow-up to Burrobot in Aquatic Ruin Zone) prefers to come out of walls to surprise Sonic. Both are more dangerous than they seem, even when the player is adequately prepared for their rushed entrance.
And for number one...how about...?
1) The Whole Metropolis Crew!?
For the love of all that is good and innocent, do not mess with these guys! How bad is this trio? Even arranging their pictures for this blog was a colossal headache! Metropolis Zone gets a fair bit of notoriety for being home to the most consistently dangerous set of badniks this side of the Death Egg. Asteron (the starfish) hides inside walls, self-destructing to shoot its spikes in all directions. They are particularly annoying as they stay inside walls, unable to be hit prematurely, they appear in large clusters and they love to strike while Sonic is stuck riding a platform with limited mobility. Shellcracker (the crab) is the least dangerous of the trio. His positioning is what makes him realize his full potential: like Skorp, he likes to hang out on the high ground, limiting Sonic's approach to getting around that giant extending claw.
Some of you may be upset that I did not pick a definitive number one for this list. Fear not! Should I have separated this 'dream team,' all still would have appeared on this list, but one stands out the most dangerous badnik: Slicer (the mantis) is a deadly ninja. He wanders the floors (and ceilings!) of Metropolis, tossing his claws the second Sonic gets near. While the claw-tossing leaves him helpless, avoid these angling projectiles is far from easy. To have a realistic chance of surviving, Sonic must literally predict his appearance, stop and slowly approach the mantis to lure him into throwing the claws at a safe distance. Approaching this badnik at even a jog spells certain doom for our hero!