As far as legends go, it's hard to compare the first-generation Mustang. From the record-breaking sales of the first Mustang pony cars in 1964, to the muscle car madness of the late 60's, to the wheezing carb-restricted big bodies of 1973, no generation of Mustang had a more dramatic transformation than the first generation.
The second generation carried the unfortunate torch and is considered the least desirable of all Mustang eras. Ford hit some runs with its performance versions like the Boss and Mach 1 Mustangs but the biggest jump is when racecar driver Caroll Shelby took the mustang from a "secretary's car" to one of the most iconic and sought after Mustangs in history. This has less to do with Mustangs and Ford for that matter and more to do with the stricter emissions due to the gas crises of the 1970s. NO manufacturer was putting out enthusiast-focused automobiles.
1964 1/2: - The inception
Shelby: - 1965-1970* (*69-70 Through Ford) GT350 /GT500
Mach 1: - 1969-1978
Boss 302 & 429: 1969 - 1970
King Cobra: 1978
The Mustang nameplate narrowly dodged a front-wheel-drive layout and instead decided to stay close to home and develop the Fox Body Mustang. The sky is the limit with these year Mustangs, consumers loved their look and racers loved their durability. The swiss army knife of Mustangs, the Fox has been used is almost every type of racing from drag to autocross, to drifting, and with cheap parts to boot this was definitely the working man's Mustang.
Four Eyes - 1979-1985
Saleen - 1984- 1993
Cobra R - 1993
SVO - 1984-1986
This era may best be described as a mixed bag. The SN-95 Mustang was the successor to the Fox and while some championed it, others were left scratching their head. The 1994-95 model years were actually the perfect year for folks that loved their Foxes but also didn't mind an aggressively fresh redesign. You still got the 5.0 under the hood and if you hated the Vanilla Ice Fox styling then you were in luck. In 1996 the line in the sand was clearly drawn. The new engine offered for the SN-95 was the high tech 4.6 L modular engine that was originally used in the Lincoln Town Car, which had the old schoolers in arms and the new schoolers high diving. The SN-95 went through it's biggest redesign in 1999 with the "New Edge" Mustangs, where the car got sharper lines and eventually more power.
Cobra R: - 1995, 2000
Saleen: 1994 - 2004
Mach 1: 2003 -2004
Terminator: Cobra 2003-2004
Ford goes back in time (design-wise)! With Ford's success with the retro-styled 2003 Thunderbird, Ford knew they were on to something. The 2005 Mustang burst out onto the scene with a vengeance, pony fans were excited to see the Mustang look like the one their dad had even if it still had the 4.6 modular engine under the hood. The beefier Mustang got some shots of adrenaline with the return of the Shelby GT500, a supercharged 5.4 Liter behemoth.
The next big milestone was the return of the 5.0 Mustang in 2011. This unlike the last 5.0, was a true 5 liters of displacement, unlike the 4.9L of the old 5.0. Power jumped to a 412 hp. After another facelift in 2015 the Mustang has been very experimental. Ford now offers an Ecoboosted 4 cylinder Mustang, which we haven't seen since the SVO days. Ford also announced a new E-Mustang which is you guessed, electric. Here's to the next 55 years!