Personal Preference aside?
I'd say X-com... if only because it handles alien invasions better then anything else made. Except maybe some books I haven't read.
It seems like the most "realistic" approach to alien hunting ever and creates a super unique gameplay method because the scenario needs a unique gameplay method.
As it is though i've yet to fully get into it since i just bought it recently and i'm overwhelmed by the options and finding a good starting strategy.