Is the best D6 game the old West End Games Star Wars?   Or is it evolved versions of that game like Mini-6 Bare Bones (and free)?
https://www.drivethrurpg.com/m/product/144558

Or there is this other game I found called Warbirds that is about dieselpunk mercenary pilots operating off airborne escort carriers on a world where even the lands float in the air.  https://www.drivethrurpg.com/m/product/115960


I also have a totally free fantasy rules lite game called Pocket Fantasy that has all you need to play a fantasy setting.
https://www.drivethrurpg.com/m/product/189191

And there is the Tiny-D6 series of games.  They cover a lot, but lean a bit toward being too rules lite as well.  They are made with kids in mind after all.

I like all of those games.  But I'm always on the hunt for more.

WEGs D6. Hands down.  Rules light versions might be too light.

GURPS of course, a bit too lite for my tastes but well researched and mostly well supported.

Ghostbusters 1st edition published designed by Chaosium and published by West End Games. Having Greg Stafford, Sandy Petersen and Lynn Willis write that game was like having Alfred Hitchcock, Akira Kurosawa and John Ford direct the movie. More than three and a half decades old, and still the best intro rpg ever made.

WEG d6 Star Wars is hard to beat, the only one that uses just d6 and supersedes it is Traveller.

Best for what?

There's OpenD6 (WEGs Ghostbusters & Star Wars but OGL) It has Fantasy, Space and Adventure (Pulp Indiana Jones style).

There's also Cepheus Engine (Traveller but OGL) and the miriad of different genres adaptations it has.

Edited to add:

Can't believe Barbarians of Lemuria hasn't been mentioned.

I'm gonna toss in another vote for Ghostbusters, because it very effectively covers the setting and is easy to learn and to explain to others.

And the Ghost Die mechanic is awesome.

As others have said, WEG Star Wars (or, if you prefer, Ghostbusters and its derivatives), or Champions/Hero Sysytem. GURPS. Traveller. I'd throw an honorable mention to Tunnels & Trolls here as well.

There are a lot of great D6-only systems, but "best" is pretty narrow, so I won't list all my favorites, just those that have stood the test of time. I'd say the games listed above have large enough fan bases, and have sold enough products, to count as the "best" D6 systems.

Mini-6 Bare Bones is good and bad in my opinion.

Good
1.  Simplified combat.  The opposed roll system is there if you want it, but the default alternative method is to pre-calculate your defense rolls as an average. Also, the parry and block skills are gone.  Instead, you use your same skill in both offense and defense rolls.  They did keep dodging vs shooting, though.  Defending is no longer an action.  So if you want to shoot twice and dodge with a -2D6 to all of three skill rolls, now you just say you shoot twice with a -1D6 to both shots, and your static dodge number is what it is, unaffected, and always in effect.  It's honestly an Armor Class by another name.  It's done to avoid confusion, and to speed up gameplay by halving the number of dice rolls made during combat. 

2.  Simplified ability/attributes.  Four instead of six of them.  Two physical, and two mental. 

3.  Toned down Force/Hero Points.  They're not as powerful, but have more things they can be used for. 

4.  Setting agnostic.  Star Wars is cool, but any setting could be fun.  I'm planning a swashbuckling adventure in the age of sail. 

Bad:
1.  Poorly implemented Magic system.  At the very least the penalty for failure is too high.  I house ruled the penalty for failing a Magic skill use is a -1D6 to just that power you just tried to use, not everything.  Also, I use GM set difficulty numbers and effect sizes.  Bending a spoon with telekinesis is very easy, but lifting an X-wing out of the swamp is hard. 

2.  Boring melee combat.  I've fixed this when doing 1vs1 fights between important villains and my players by using an optional rule called Dueling Blades from Griffon Publishing.

3.  A Star Wars setting that has a power that doesn't work.  I've replaced it with a power that only effects ranged combat by treating a knights plasma sword as 360 degree cover while the power is active.  That works better. 

I'm also tempted to make my own Supplement Book for the game that is full of optional magic systems.  Plural.  Use the system you want, or even all of them.  What that game has now is terrible, something even the stutters admit in an FAQ.  They ran out of time as their business went under and shipped it with what they had.  Now it's just not worth the time to fix. 

Bad. 

I am embracing playing and running D&D my way.

Everything is based upon d6 rolls. 
Either 1d6 or 2d6, for all resolution mechanics. 
Hit Points for PCs, NPCs, and Monsters; are based upon d6s.
All Damage is based upon d6s.
Target Numbers are either a 4, 5, or 6.
I use Advantage / Disadvantage a lot.
The Mighty d6 can do it all.

Of the games I know that only use d6, my favorite is Shadowrun 1E.  That probably has more to do with the setting than the mechanics though, to be totally honest.

Quote from: Jam The MF on September 05, 2022, 05:50:37 PM
I am embracing playing and running D&D my way.

Everything is based upon d6 rolls. 
Either 1d6 or 2d6, for all resolution mechanics. 
Hit Points for PCs, NPCs, and Monsters; are based upon d6s.
All Damage is based upon d6s.
Target Numbers are either a 4, 5, or 6.
I use Advantage / Disadvantage a lot.
The Mighty d6 can do it all.

I want to know more!

Quote from: Jason Coplen on September 05, 2022, 07:43:50 PM

Quote from: Jam The MF on September 05, 2022, 05:50:37 PM
I am embracing playing and running D&D my way.

Everything is based upon d6 rolls. 
Either 1d6 or 2d6, for all resolution mechanics. 
Hit Points for PCs, NPCs, and Monsters; are based upon d6s.
All Damage is based upon d6s.
Target Numbers are either a 4, 5, or 6.
I use Advantage / Disadvantage a lot.
The Mighty d6 can do it all.

I want to know more!

Microlite74 3d6 edition, and it's free to download, it's not what Jam is doing tho.

Everywhen, same underlying system as Barbarians of Lemuria, several settings included IIRC.

Blades in the Dark, Star Wars d6, HERO, EABA - in no particular order.