That depends on the type of clan you are after - there are 2 main types of "Clans" or "Families" in Mafia Wars - "social" and "fighting".
SOCIAL CLANS
Often referred to as "add me" clans because of constant requests for additional mafia these type of clans suit players that aren't really interested in clan wars, but more interested in doing jobs and completing collections while hanging out with friends.
FIGHTING CLANS
Check your mafia - you may find someone you respect is a member of a clan and they can offer advice, or confirm if the clan you are cinsidering has a good reputation.
Stronger clans (RYFG, Mutants, Killer Clowns etc) have 2500 attack 2500 defense skill and above requirements, other well respected clans (eg LCN, Vipers, DE) have 1250 attack and 1250 defence skill requirements but will provide training if you are minimum 1000/1000 skills
Facebook Clans:
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It depends on what you mean by the best...the best may not always be the strongest!
Armageddon/4H
DYY
Elite
[SIN]
Cupola
Delirium
Fight Club
IndoMafia
Garuda
The KISS of DEATH
ITA
VGL
LCN
LxM
OG
OMERTA
LAW
MUTANTS
Dby
DIECI
Ares
NB(OG)
[MWI]
[Legion]
R.I.P.
FUBAR
Delta 9
TPK
MMA
[Infinity]
{f♀f} Femme ♀ Furies (Ladies only)
...to name only a few.
A long list is available in the associated links. I would suggest looking into several of them and choose the one that best suits your needs.
Chat with our AI personalities
ARMG, RYFG, MUTANTS and Delerium all are stronger clans than Elite now days and are probably the strongest clans in mafia wars. These clans require 4000 attack+ 4000 Defense minimum to join (and much higher for RYFG but fewer RYFG players), compared to 500 attack/defense for Elite
[Elite]
[Elite] was the second clan ever formed in Mafia Wars. They chose simply the name Elite because they were the elite, the best, in Mafia Wars. The tag they chose to identify their characters was [Elite]. Over the years they have kept the name and the tag pure.
Many clans have sprung up using the name Elite at either the beginning or middle or end of their name but still Elite has remained supreme as one of the foremost leaders in the Mafia Wars game and while it has from time to time had to request and sometimes force another clan to change their name if it infringed too much on theirs in causing confusion as to it being an extension of the Elite clan; overall, Elite has been more than generous in allowing others to use the prestigious Elite name when forming their clans.
There is only one true [Elite] and naturally this is the first Elite ever formed. Elite goes out of its way to help newcomers in the game and asks for only one thing in return: Loyalty and Respect!
Many have been jealous of Elite and have accused them of cheating and using scripts to get ahead. It is unfortunate that some will use these methods to get ahead in this game but it has never been condoned by the administration of Elite. If anything, Elite has gone out of their way to discourage and expose those who would blatantly cheat to get ahead.
Unlike most other established clans, Elite allows lower levels to join their clan and does all it can to help them to build their character.
Elite is...well...Elite!
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New Blood-Original Gangsters, [NB[OG] is a highly organized Mafia group in Mafia Wars by Zynga. This Mafia/ Family is rooted from the once glorious first and foremost group ever created in mafia wars called the OG Mafia.With a new foundation and new administration, we have made a hybrid group combining the new and the old ways of the game, turning the once-known OG Mafia to your new and improved New Blood OG Mafia. They are all low level players and can never beat Elite. Elite has the biggest cheats and stolen loot makering in the game. I myself was an Elite but have stopped playing because I lost my job and can not afford to buy more stolen GF points.
P.S.
Elite has been defeated several times by [NB[OG].
Disclaimer: Please note that this answer is completely biased and contains the opinions of clan members/owners
New Blood Original Gangsters (NBOG) are the oldest clan in mafia wars. They were the first to organize and the first to be recognized as a clan by Zynga. Basically, they started the whole clan thing going. Elite and RYFG are others. These are first generation clans, the second generation clans are also old, and follow only a few months behind. These include clans like Lycans, CIA, , FHL, ICE, SSB, and a slew of about three dozen other "old-school" clans. After these, clans began to pop up left and right, until today they number in the hundreds.
Elite in the beginning, then it was [LCN]...now its ARMG, RYFG, Mutants and probably DELERIUM