When you say "hold thier value" it depends on what type of timeframe we're talking about.
I like Spike's and BCM, but they haven't been around for a long time and are unproven. That, and they don't enjoy the mass of name recognition among a large amount of potential gun buyers that would otherwise drive prices up. Another aspect that might cause a gun to hold value would be scarcity.
As among the tops in investment rifles would be Colt. They have wide recognition and have a track record of being able to hold value. Knights Armament might as well as they are a military contractor and have special edition rifles that would be desired among collectors (though I might keep some of thier rifles NIB if holding value was my primary goal). If by chance another AWB does come down, it really won't matter what brand it is, (even Vulcan!) prices will increase as the pre-ban firearms will be sought due to thier status as grandfathered in.
I personally wouldn't ever buy a rifle solely as an investment, (mostly because I like to shoot the damn thing) but what to look for would be recognition, track record, special features, or scarcity.