Answer: [CrtD]x4 (Four Critical Severity Mods on the same weapon.)
This is simple, but people actually argue about it. There's no question, CrtDx4 and CrtDx3+Pen are clearly the best. It's a 1a/1b kind of thing. Why CrtD? Because you can get CrtH at a higher rate than CrtD from Fleet Tactical Consoles, so you should get your CrtD from your weapons and your CrtH from the consoles. Plus, we all know to use Romulans, right? By getting MASSIVE bonuses to both critical chance and critical severity, your damage will go through the roof! This is best accomplished with ship weapons that have high [CrtD]. Get as much CrtD on your weapons as you can - we can't all afford the perfect weapons, so get the best you can afford. CrtDx2 or x3 is better than nothing.
Advanced Tip: There are people that will say CrtH (Crit chance) or Acc (Accuracy) mods are better, but they are simply wrong. First off, getting lots of Acc is really bad in PvE (Player vs Environment) combat. Why? You don't need accuracy in PvE - the enemies are easy to hit. Not only that, but FAW (Fire at Will) doesn't properly use excess accuracy - it's actually a known Bug that Cryptic hasn't fixed since an employee acknowledged it 2012 and they likely never will fix it. (Typically, Accuracy above what would be 100% chance to hit is translated to CrtD at a fixed rate for every 10% accuracy above 100%. With FAW, you get virtually no benefit from that excess accuracy.) FAW is really the best energy weapon firing mode for pure DPS numbers, so it really makes Acc incompatible with many high-damage builds. You could make an argument for its use in PvP (Player versus Player), but it's just not that good. CrtH is important, but you should be getting that from the Tactical Fleet consoles, not your weapons. Remember, you need to get BOTH Crit Chance and Crit Damage to maximize DPS.