I have watched every Gangland episode (haha!), besides studying organized crime in general.
Everybody can be scary, but when you add up the level of ruthlessness, organization, etc., and subtract for level of traitors and snitches, it's MS-13, and it's not really close.
To sum up: everybody can get crazy now and then, but MS-13 is completely loco, and has very few traitors that see many days outside of the organization.
The Mafia had their day, but they have hundreds of turncoats on record, and many more not on record. The same with Bloods, Crips, Hell's Angels, etc.