For me, it's motion spotter 5. I put it off forever because 1,000 certs seemed excessive, but man, was I wrong. Set one of these down and it's pretty much a maphack for the whole base. It's paying for itself, too. The other day at a tower fight, I stuck one at the base of the tower every few minutes, barely killed anyone, and came away with over 18,000 XP (with a 0% modifier, no less) in a 20-30 minute fight.
Munitions Pouch for engineer. 5 tanks mines, 3 proxy mines and you've essentially got a one man base defense ready to go. Combine that with 4x c4 when you're operating with an outfit against regular MAX crashes and you've got a suit slot that makes engineer not only fun to play, but useful outside of MAX babysitting duties.
Recon darts are even more multiplicative. The final upgrade is something like each of your darts having more radius, carrying more darts total, and the darts lasting longer. Personally I find darts far preferable to sensors due to wanting to know what's in a building before I get there (sensors provide very limited value when attacking, basically requiring you to get right up to the building you want to scout before you can get intel on it, while darts provide value wherever you want and the only drawback is you have to refresh them more often.) AI noseguns with thermals become completely ridiculous. Thermal sights help a lot of weapons, but when you're hunting infantry with an AI nosegun thermals are basically brokenly overpowered (and honestly there should be a defensive slot for infantry, and one for vehicles, to let you counter thermals.) Grinders w/ Slugs are nuts. I used to think Mattocks were the only useful NC weapon, but slugs seem to be this stupid "take this or you're doing it wrong" mistake that's enabled for NC players which isn't present for the other factions.
2 things. First would be putting certs into and using the C75 Viper. My first day in game with my wallet fat with SC, the Depot had the Python HE on on sale and I never gave the Viper a chance til a couple months ago. It actually became my go-to and very first Auraxium. Second would be Sticky Grenades. Again, first day in game I accidentally hit G in a crowd of friendlies and swore off grenades immediately. A couple months ago I had a bunch of certs piled up and decided to try them out. Dear God, most fun ever.
I'm enjoying my time as a AI harasser driver so I wish I'd tried that out before dumping my certs into heavy & medic which I don't often play on the character.
Serpent with 1x Arrow sight. SERIOUSLY, the thing is SO good, and there is so little deviations, I LOVE IT
(F)Armistice/Lynx, great ADS guns once you get the recoil figured out. Also the Striker, maybe I could've enjoyed it's glory days if I'd gotten it sooner.
Guess I'll be certing into that next. I used to cry about skillsuits but pulling them myself is so therapeutic.
I used to use the Gauss SAW a lot, but ended up just using other guns because I was such a terrible shot and it's awful at CQC/somewhat medium ranges. Came back with a bunch of certs in my pocket, and gave it a compensator, HVA, Reflex sight and an advanced foregrip. Higby on ice, that thing, when used for single or burst-fire at long range, is like a sniper rifle! I've countered clueless people from range with that sucker, and headshots too! Still kinda sucks that it has the lowest RPM of all the LMGs (I think) and kicks like an Auraxian mule.
Rocket pods and a better computer. Could have spent the first month farming infantry to get an extra 5,000 certs. I stupidly grinded my way to certing pods instead of buying them...only to get the splash nerf a week after I got it.
