What is the best caliber in AR15? First lets assume we are talking rifle because 9mm is a pistol caliber.
Also when you say 7.62 which one? There are many.
IMO if you do not have any AR's it makes sense to have the parent 5.56 due to popularity, the best assortment of ammunition in value/bulk/suplus packs, hunting and tactical. The hunting is somehoe limited but the 60gr nosler partition and the barnes TSX 70gr are great white tail rounds. The problem is that this caliber for an inexplicable reason is not allowed in some places for deer so you need to step it up.
The next reasonable choice is a 6.8SPC2 and this is a great white tail, pig, black bear carrige but more expensive to feed so reloading is a good option.
Also the 7.62 russian (Same as the zastava) is a great cartrige for the AR15 that can shoot both value packs of steel or match grade loads for both target or hunting. The bullet assortment is not as rich as any of the other whisper or blackout due to the 310 bore but it yields more power than the blackout. Also great subsonic with heavy 303 british bullets. All this 308'ish have plenty of punch at short ranges make them great for white tail and other critters but loose power pronto due the low ballistics coeficients involved. So if you want to use it for lets say 300 yards better stay a couple of calibers down or the 6.8 case all together.
You also have the 30 Remington AR in 308 in single stack that will pop large game at very considerable ranges.
I am currently working in a 6.5 necked up from a 6br norma lapua that I will be using with big game and long range that is giving performance superior to other larger cases and calibers but this is a wildcat, expensive and it is not for everyone to reload.
Another very reasonable choice is the 6x45. Great performance same brass but ammo is expensive and hard to find so better you need to reload.
The list goes on with endless options from big bore Beowolf, socom, 50AE, etc... to WSSMs and even shotgun for the AR15 but
you have to think what are you going to do. Even if you are going to hunt makes sense to have a regular 5.56 carbine and then a nice upper for hunting that you will be shooting maybe less.
The zastava PAP are great carbines. The Yugoslavian always had good steel and it is very reliable like any AK. The issue is that it is single stack so you are restricted to the 10 rounders. It can be converted to double stack but you have to send it in therefore offsetting the original attractiveness in the price.