Mangaka is pretty clean and 20xx is cool too

Imo for fennec Neo-LL37 For octane WWE Roman Reigns For dominus Suji and for any other car it is either Sunsetter or Interstellar (of course if you choose the right colours).

Storm watch

And then any universal decal I'd probably say bubbly but I think that it's kinda underrated and I'm probably the only one who ever uses it. It goes hard with classic goal EXSPLOSION, crimson octane, standard, black dieci and classic trail. Give it a try :P

Dominus Suji or Or-aise

Fennec. Edge burst

Octane lone wolf

the sakura octane one is tough

Octane: kana - titanium white Its so flipping clean dude

That's like asking what the best color, or the best ingredient is. Very much depends on the type of vehicle, color choice based on the other items, as well as taste of the person creating it. Some of my favourite cars use decals that just come with the vehicle by default, like Circuit Pro for the Maverick. The decal I used the most is probably RLCS, I got a bunch of them for a few cars and they work quite nicely. Besides them, I also used 20XX and Gale-Fire a number of times.