I voted Ruin simply because it does three environmental effects including slag to other enemies. But Helios and Chain reaction are tied at second close second. Personally these are the "big" three IMO for a Maya build because they go hand/hand and make Phase Lock the destructive force what it is.
The other skills although valuable in this are nice but are much more situational skills and or/stat boosters re reload/speed magazine or effects while in fight for life etc. Some of the skills are actually negated quickly by the big three where skill points could be better used somewhere else because between the "big three being used" than shooting enemies one quickly destroys the enemies not leaving much use for the skills. That stat boosts of the other skills can also be offset some via BAR.
But Ruin is probably the best of the big three in this skill tree by a slim marging over Helios/chain reaction IMO.