Pretty sure Articuno still sucks on its own. It counters 2 common threats (I think Dragonite is unlikely to be common/threatening) and does very little else. I can see it not sucking AS much as it would in OU since it can hit more things with the likely absence of Suicune/Cloyster, but its few options are just not as good as your other choices.
Though looking at the damage calcs, if you assume Lugia carries Thunder (as you are saying), Lugia kills Articuno faster than Articuno kills Lugia, plus it gets access to Recover, so it might not be the most solid answer, depending on things.
Explosion might not be the heaviest influence in the game. At first glance, Explosion just looks like it does in OU; if you plan it well, consecutive/well-planned Explosions can make holes for powerful sweepers.