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This thread will turn into a shitflinging hellhole because the one retard showed up, but I will repeat that SHF's take is the best one. Yes it's based on a terrible movie and the design itself won't please everyone as it blends several different xeno designs together, but in terms of posability and sculpt, it's the best. The second mouth has some issues coming in and out properly, least on my copy, but the engineering is fantastic and you can get a shitload of great poses out of it. Only problem is it's hard to find and goes for alot on secondary markets.
Neca has issues with QC and while there are more versions than anything else, they're not engineered as well. Revoltech poses fantastically but alot of people won't like the colours. I haven't tried them myself but Hiya also makes 1:18th xenos of basically every variety you could imagine, as well as Marines and Predators for fun with displaying all of them.