Streets of Rage was a Double Dragon clone.
If "Streets of Rage" was a "Double Dragon clone" then a Lamborghini Murcielago is a Toyota Camry clone.
Double Dragon didn't have:
1) Team up moves (throw your partner as a projectile)
2) multi-move "specials" (not just one)
3) Trade-life-for-invulnerability specials (Final Fight did, but only 1 special per char)
4) make-your-own combos (try chaining a 2-hit jump kick move in a standing combo in DD!).
Just because two games are of the same genre doesn't make them clones. Tekken is not a Street Fighter clone. Daytona is not a Pit Stop clone. They are the same genre, but light years apart in terms of gameplay.
IMHO, Streets of Rage II was the best beat 'em up of all time. It had more moves, better characters (user-playabe and enemies) and a wider variety of weapons use than any other game. SoR came a close second. The third was just a rehash, and not worth more than 1 or 2 plays for novelty value. I know plenty of people who scoff at SoR for being console-only, but it's the perfect example of superior gameplay over flashy graphics and bling-factor.
Final Fight and Double Dragon are absolutely without a doubt classics in the genre. But neither had the depth of gameplay to the SoR family. They are too simplistic in comparison. That's fine if you like simple games, but I find them tiring after one play. Having mastered all the techniques of a game in the first level, there's nothing more to do. SoR2 gives you mutliple characters all with a HUGE range of moves for a side-scrolling beat'em'up.
TMNT to me was a gimmick. The hit detection sucked, and gameplay was missing completely. The only thing that made it fun to play was the TMNT branding. Likewise for "Simpsons" which was the same game with different sprites.
Just my 2 cents. My old MegaDrive (Genesis for the US audience) still gets SoR2 play today, over 12 years later (I bought the game only days after official release). I can't say the same for any other beat'em'up I've ever played.