From a pure numbers perspective full aggressive with light armor seems to give the highest dps as we cannot see the actual dps changes of other gear traits nor the actual stats of the companions.
IMO I don't think its about it being the strongest combination (although it is up there) but the easiest to see good results with.
Crit is RNG based and without knowing the companions baseline and when it actually crits makes it a 'this may help but hard to tell'
Pen falls into that same category of not knowing what their baseline is. One could stack pen till we no longer see a rise in dps on overland mobs and get a rough idea but doubts it would outperform a flat percentage buff to dmg.
Reducing Ability cooldown relies on the skills you pick as longer cooldowns benefit more than short. A flavorful balancing on the hind end of a dps build for sure.
TDLR: Aggressive is the easiest and most noticeable difference in DPS until more information is gleaned off companions. More optimization of min/max will come as more details are found out. Till then hunt for the aggressive, pray to the rng gods, and hold them over with the rarer parts.
All in all.... they are mortals running around with a soulless husk that repeatedly fights anything its suggested to, including deadric gods, while yeeting off cliffs without care because death is meaningless to us (outside occasional frustration in our own ability to smack something hard enough)
It really depends on how you build out the companion and what benefits you want out of their build. Most people look at pure damage output and that's usually what they usually find, few think outside of the box for what other builds could be of use.
The maximum penetration you can give companions is short of what Major and Minor Breach will cover to be maxed out. I gave Bastian all ability cooldown so his buffs have no more than a 2-3 second window of downtime between casts. Increased Buff/Debuff duration is crap as most of their effects last up to 8 seconds and the difference is minimal at best. Prolific is only good on Mirri as Bastian has some strict requirements to prevent accidental casting when no enemies are in range, but it also makes his somewhat less useful even when he is in range and the game prevents you from force casting it.
I debate the usefulness of increased healing as most healing spells are either cast as it comes up in rotation (whether you're in range or not) or have percentage conditions, they won't actually be smart healers where and when you need it. In that state of mind, reduced cooldown is probably better so they are casting it more frequently thus increasing the chances of actually receiving it.
I don't have enough Aggressive gear to put on bow Mirri to really compare vs. Shattering (penetration), but Shattering traits are a pretty noticeable improvement overall.
Maybe Aggressive is best but a good amount of Shattering probably comes second.
I won't claim to fully understand companions but I get better mileage from Shattering than I was getting from Focused (+Crit). The main thing bow Mirri does is light attack, and it makes those hit for their full value vs. boosting randomly.