[Energy Siphon] for lower level missions;
[Sprint Boost] for Plains of Eidolon and Orb Vallis;
[Corrosive Projection] for the rest.
Observations on stats
Strength: You only need 100% for the 100% damage resist from Kinetic Plating.
Duration: The most important stat on [Gauss], the more the better.
Efficiency: Its always good, but you dont need too much, Kinetic Plating and Energize will supply you with energy.
Range: Most builds will have low [Reach], which I don't like. It's not that it's bad, just different. Having [Reach] will allow you to use Thermal Sunder to its full potential.
Speed: Speed is good for Plains and Vallis, but it is not attractive for missions since you will not be able to control and will hit the walls all the time.
Notes on some mods
[Umbral Vitality]: Necessary for high level missions because Toxin, the most annoying type of damage, will bypass your Kinetic Plating.
[Speed Drift]: Cast speed is too good to pass on, but we can't afford to use [Natural Talent], so this mod will give you a bit of two nice stats.