My list of most intimidating gangsters:
Salvatore Leone-He was something what many people think of when they think of a mafia boss,he had power.If he didn´t try to kill Claude,he would probably still control LC.
Mikhail Faustin-Very unpredictable and crazy character.It was scary how he shot his thug that kidnapped Niko and Roman just for doing something wrong.
Dimitri Rascalov-Another unpredictable character,but unlike Faustin,he wasn´t crazy,but instead a cunning traitor.
Bulgarin-When he appeared in GTA 4 he was really scary.They did make him a little less scary in TBOGT,but it´s still crazy and scary how he shot at and maybe even killed his own sister.
Billy Grey-He was also an unpredictable traitor,and also very violent(he shot the AOD biker in front of The Lost clubhouse,he smashed that mechanic´s face...)and fearless(he participated in some attacks on AOD).
Sweet-He is experienced in crime,lives by his ideals and is not afraid to risk his life to do something(like when he jumped on a firetruck to try to stop Tenpenny or when he helped in some attacks on Ballas).
Hazer-That crazy dude stormed a projects full of armed Vagos while wielding a katana.He did get shot by the end of the shootout,but for a minor character surviving that long without a gun is pretty impressive.
most of the protagonists-I guess almost all,if not all of them did something incredibly badass(Johnny storming a prison,CJ breaking into military bases,Tommy storming a cartel mansion and later defending it from mafia...)or ruthless and scary(Trevor torturing a guy,Toni blowing up an entire neighborhood...)at some point.
My list of least intimidating gangsters:
Martin Madrazo-Leader of a cartel,surrounded by thugs,and Trevor still kidnapped his wife in his own house.
Brian Jeremy-How did that dork even make it in the badass biker gang?
Joseph Daniel O´Toole-He looked even more dorky than Brian Jeremy,but at least he was never a full member of the mafia.
Stretch-For an OG,that guy wasn´t really badass or intimidating.The only violent thing he did was killing that Ballas OG D,but even that was only to save himself.
Playboy X-He acted more like some rapper who only talks about money than a real gangster.
Jizzy B-A wannabe tough guy.The guy was just beating up hookers,but when CJ pointed a gun at him,he looked like he was going to piss his pants.