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That's a hard call; it's a toss-up between the AK 47, Galil, and AUG. I'd say the AUG, as I find it to be the most versatile. For attachments, I usually put a red dot on it, or dual mag if I'm not using sleight of hand.
My favourite was the HK21. It was cumbersome, but it was easily my favourite just cos how badass I felt lugging the thing around. After that, the Stoner63 with the ballistic knife as my secondary. That stoner got me out of some tight situations!
For me, it was the Enfield. It was such a bad gun and nobody ever used it, which is why I liked using it a lot. I like the way it looks, it looked like an L85A1 or something, and I like that gun. But I used it mostly because nobody else wanted to use it, because the recoil was a bit weird. Anyone else has used this gun? What attachments do you like on it? I used the ACOG sight (don't remember how it was actually called) and the suppresor on it.