I know for Tac the speed loader card is great to shoot booms with no reload
Jac has the card that explodes to 130% radius at level 6, speaking of this card it used to list the damage but at lvl 6 it only mentions the radius, did it remove the damage and replace it for radius?
Are there other cards that are so much better when they hit lvl 6?
Thrill of the Hunt
Execution shield
Longshot Handling
Brutal Claw
Score boost and epic score boost for all classes.
That’s… Pretty much it.
No, i mainly just earn cards through frenzy matches but if there are specific cards that do best at 6 i would just purchase it with gold, i play all the classes
Hmmm Needs to see the cards to say for sure which ones but off the top of my head
Gunner - Serrated Edge, Reflective Shredder and Bait Armor (I think that’s their names)
Combat Medic - Intervention
Blademaster - Thrill of The hunt
Brawler - All health and damage resistance cards lol.
Nomad - Consecutive Shot
Can’t think of any more right now, but those are the cards I bring in every build I use.
Any card that is not heads up.
Good god you play all classes? More power to you. I’m too boring for all that. I’ll give you my thoughts on my favorites that I personally would want to be level 6. Not saying they are the best necessarily.
Nomad-Execution Shield
BM-Venom Blade
Pilot-Cold Finish
Anchor-Bloody shot or barrier feedback
Demo-Razor Hail (by default)
Cards that should be level 6 asap:
Longshot Handling - overpowered card at 6
Execution Shield - Survivability
Resupply Amplifier - huge difference at 6
Resupply Speed Loader - reload bypass
Bait Armor - survivability, cheap to upgrade
Bloody Knife - damage output, cheap upgrade
Stim Siphon - Survivability, cheap upgrade
Bloody instrument - damage output, cheap upgrade
Perk Up - Survivability, cheap upgrade
Damage Dash - Survivability, cheap upgrade
Reaper - Survivability, cheap upgrade
Enhanced Stim - Survivability, cheap upgrade
Bloody Shot - damage output, cheap upgrade
There are many other Purples that I think are worth spending coins on, albeit they are expensive upgrades and function okay without being maxed:
Shredder (explosive bleed), Stim Batteries, Thrill of the Hunt, Killing Time, Critical Parade, Consecutive Shot, Team Resist, Efficient Fabrication
In terms of major change from level 5 to level 6, pretty much any card that has anything to do with damage resistance (even if that means a difference of only 2%). The way damage resistance is calculated, even 2% can make a great difference.
Then there’s cards that just generally cause a great difference on level 6 compared to level 5. A few examples:
Longshot Handling (100% guaranteed automatic reload is more reliable)
Resupply Speed Loader (Active bullets have a great impact on damage done to enemies)
Good Kill (those bigger ammo pouches are a lot better than simple pilfer ammo)
Probably more…
Generally I do recommend to invest your coins into Epic cards though. Those are the hardest to get to and upgrade. They usually take the longest (even Legendary cards are usually maxed out faster).
Here’s at least one that you must need for each class or at least highly recommended.
Blademaster - Venom Blade for escape only, Thrill of the Hunt, Electro Blade
Demo - Shredder
Infil - Reaper
Marksman - Icy Precision, Longshot Handling, Critical Parade, Exploit Weakness, Modified Longshot, Ambush,
Nomad - Execution Shield
Architect - Repair Efficiency
Protecotor - Big Knife and Bloody Blade
Slugger - Power Shot
Striker - First Strike
Combat Medic - Overdoing It
Mechanic - Repair Efficiency
Robotics Expert - Combat Engineer
Tactician - Recharge Bounty and Shredder
Anchor - Adrenaline Junkie and Boltok Bandolier for Escape only, Bloody Shot for both escape and horde
Brawler - Perk Up
Gunner - Bait Armor, Heavy Capacity
Pilot - Healing Explosives
Veteran - Focus and Headshot Master
You could also put “dug in” in that list to, adds tankiness from snipers and other resources to. I rarely see randoms using that card, hence, why they go down so easily.
Also, it’s a green card, dosent cost much to fully upgrade it.
PASS
I am disappointed in you listing this card but not The Hammer.
Especially since I never use Healing Explosives and get by just fine regardless. Not that I’d judge anyone who does use it.
Ones that you use.
One of the most underrated Cards (combined with the Hammer or a Boomshot) for Pilot, the main Reason why i was able to do a Solo/got to wave 40 plenty of times on Allfathers Arena.
Most overrated Card for Pilot is Cold Finish.
Healing Explosives + Aggressive Armor is one of the driving factors, for me, for Pilot being the best class in the game. For Horde.
And the Hammer. Crazy how 3 Cards can make you a Tank while stunning/Killing everything. 
I don’t exactly count solo Horde as a way to judge cards under regular circumstances. I just don’t use it because firing the Dropshot already requires little exposure as it is, on top of it providing a stun effect with Pilot, and since I don’t tend to use the SB Mulcher much, I don’t really run into self-healing issues with it. But if you get value out of it, great.
Also; Cold Finish got devalued a lot with the various balance changes. I remember it being very sought after before we got dailies and enemies lost 20% of their HP on Master. Especially for bosses.
I used Solo-Horde as an example to proof my Point that HE is crazy underrated for ANY type of Match.
There was been multiple Matches were i was last Man Standing/not DBNO with no Ult. and that healing and Stim on top of that with AA saved the Day.
Cold Finish was a good Card when using a Trishot as Pilot was “the right way” to play.