Q) Which Assault Rifle is best ?
This question can be interpreted in two ways, so I will answer both.
1) If you are asking which Assault Rifle I prefer, i.e. the AK47 (7.62x39), AR-15 (223/556), or the M-14 NATO (7.62x51, 308), my personal choice is the AK-47, as long as it was made in one of the former Soviet Bloc countries. The AK-47 is a flawless shooting rifle with almost no issues or problems. The typical and popular U.S. Made AR-15 in 223/5.56 has been around for almost 50-years and it still has issues, mainly with jamming. As for the M-14, 308 NATO rifle it is bulky, heavy, and more expensive to shoot.
2) If your question was centered around which is the BEST Assault Rifle by Manufacturer, that is more of a personal preference. It's like Ruger versus Colt, both have their following.
Personally I own in 223/556 a DPMS 2013 Panther, and a 2012 Stag Arms.
I also own a Czeslovakian made AK-47 7.62x39