The scariest characters are the Detective Boys.
1. In real life, children at their age usually would have been in serious mental disorder, paranoia, schizophrenia... etc. after witnessing that many scary murders, bloody corpses, ruthless murderers, kidnappers, bomb explosions... especially when sometimes those events almost took their life. The Detective Boys, however, are "immune" to every of these. Also, although rarely, sometimes they don't even scream when seeing a corpse.
2. They don't seem to be afraid of directly touching bombs and move/hold them (as shown in both "Bus Hijacking" and "Red Robbery"), something even grown adults do not dare to do if they're not experienced. I wonder do their parents know about this ? I guess not.
3. They (together with Yamamura) are sometimes considered as the BO "Omen" in fan communities. Many cases featuring them were plot-related, or a case that marks plot development ("Pisco Case", "Bus Hijacking", "White, Red and Yellow" "Red Robbery"... and some cases not yet aired in anime). Before the 500 BO arc began, they'd made a very random appearance, and right after that file we got to see black crows gathering around Kogoro Detective Agency. File 599 is also a random DB case, but right next to that were Gin and Vodka.
4. The Detective Boys are Gosho's MOST DANGEROUS WEAPON for TROLLING