ARAH with Rise Of Cobra coming up second.
Real American Hero gave us groundbreaking figure construction (granted it had been used before by lesser lines, but never with SWIVEL ARMS or ball-jointed heads!), a metric buttload of characters (in 12 years, we got fewer Snake-Eyes figures then than we get in a year now
), and even when they were doing remakes of characters, they had the decency to make them look considerably different from the originals (see: Snake-Eyes 83, 85, 89, 91 and 93), but the big focus was on new characters. There was also always a great attention to detail, right down to picking the right accessories. While they didn't have jointed ankles or wrists, they still did things best compared to later lines.
The accompanying fiction varied... the comic was wonderful and never talked down to its audience while featuring great character interaction. The cartoon? Even as a kid I felt insulted at looking at this "highly trained special mission force" acting like goofballs 90% of the time... but it did manage a few noteworthy episodes like There's No Place Like Springfield, and at least it isn't Resolute.
Everything between GIJoe Vs Cobra and DTC was crap. Ridiculously wonky proportions all around, undetailed head sculpts, random choice of accessories and poor bios all around.
The accompanying fiction, while still no Resolute, was really poorly written.
Sigma Six was great in concept, the problem was the cartoon was quite subpar and the figures were too expensive to get into. They lacked customizing potential too.
25th Anniversary... was just ARAH designs with even more parts reuse abuse (it took until ZAP came out before Original 13 characters got different legs from Snake-Eyes!!) and a new articulation style. New figures like the Para-Viper were great but they were just a needle in a haystack.
Resolute... Ugh. For a cartoon aimed at "older audiences" it was more immature than children's cartoons like Ben 10 or Transformers Animated. Oh, sure, you had deaths, but personalities got de-evolved to lower levels than they had even in the 1980s cartoon, the plot fell apart after a third or so (maybe even a quarter) of the webisodes and it featured wallbanger scenes like Duke and Scarlett standing right in the middle of a crossfire without getting hit or having to dodge fire once, and Duke figuring out how to reroute the doomsday weapon in five seconds.
In terms of writing quality, this thing ties with DiC as the worst GIJoe's gotten,
ever.
The few figures they made for it were decent, although they went apenuts with elbow pads and knee pads.
Rise Of Cobra comes in second. There's that same level of attention to design and weapons (except the missile launchers, obviously), and it has the expanded articulation of 25th Anniversary, but sadly the line has flaws that prohibit it from reaching ARAH's level of greatness.
The line is still in infancy, so there just isn't as huge a choice of characters to go for. The vehicle drivers lack in quality, often sporting the very strict minimum in terms of paintjob. A lot of the female head sculpts are just icky (although to be fair, ARAH had that same problem
).
But overall, considering how wonderful a lot of the figures out and a lot of upcoming figures are, coupled with the great new things they're trying (compared to 25A's "same as the 80s but with a different construction") makes it come a distinct second in my favorite eras.
The movie rocked too, that certainly helps!