Dboys is my pick from that list. Their externals aren't the best looking, but they are built well and will last quite some time if maintained correctly. I don't have personal experience with CYMAs M4s, but they make some damn fine AK clones. AGM is kind of "middle of the road". Not the worse, not the best, just average.
If you can save up a bit more, G&P makes a great replica. My Defender is bone stock (save the TBB and bucking) and is still going strong 5 years later. Their externals are also some of the best for the price range. Some complain of their internals, but of the 2 G&Ps I bought new, they have lasted quite a long time (my Mk46 blew up a gearbox after 200k+ rounds.) Of course, that's immaterial if you're not concerned about looks, or you plan on painting the gun.
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Finally, Summer Slaughter is coming up in less than a month, and there are ALWAYS great deals on M4s and other guns. More often than not, you'll find someone selling the same gun you're looking at, only with more mags/accessories/batteries/hookers for around the same price. Buying used can be hit or miss which is a downside, but a great way to get started without blowing a bunch of money. Since you're after a model that everyone has/had, you'll have plenty of options to choose from. You can also find a better brand for around the same price.
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