Gold almost exclusively.
I don't do silver unless a friend asks me to - too many scoremongers runining the game for the new-ish players for whom silver is meant.
I don't like platinum much, there are a few interesting waves here and there, but it mostly turns into a dps race, plus there is very little variety in the kits deployed there. I don't mind playing it with friends, but I don't pug it as a rule. Also, I am not good enough to try wacky stuff myself 
I really enjoy gold: good balance of mooks and boss units, all builds/guns are viable, and most can be made fun. A team that is too strong ruins the challenge somewhat, but the match will go by fast. The other side of the coin is that I often get lobbies with newcomers, so that deploying strong kits/weapon combos there doesn't feel too bad 
I like the Viper. I just thought the general opinion of it on the BSN was pretty low?
It works on Gold (what doesn't?), but on Silver it can absolutely destroy.
I love the Viper on Gold (on kits that increase its RoF)
on the GI, it just destroys everything, while at the same time curbing the GI's power somewhat. I also enjoy it on the GE: it still trashes shield-stripped mooks, and it applies ammo effects well enough on bosses.
^^ I am joining you...by this friday.. I ordered my copy of ME3 for PC last thursday..I will see you this weekend...
We aren't exactly in the same time zone, but feel free to add me as well! I don't run mods, but I will gladly help you grind your manifest 