Best company EVER!

My first twice flies you
My second twice puzzles you
My third twice measures you
My fourth twice tests you

ASCII from my first,
Shaped like my first,
Anchored on my first,
Misses 1 of my first

Catching a loss,
Taking two more,

When filled as so,
My third row's a scheme;
And then, my end,
A French magazine

My first remains quite solitary;
Like I too it has matches nary
My products made can be quite scary;
When defective, effects may vary

Who am I?

I think you are:

ACME - the generic company from Looney Tunes.

My first twice flies you
My second twice puzzles you
My third twice measures you
My fourth twice tests you

Taking each letter twice:

AA = American Airlines (who 'fly you')
CC = Cryptic crossword (which 'puzzles you')
MM = Millimetre (which 'measures you')
EE = Entrance exam (which 'tests you')

ASCII from my first,
Shaped like my first,
Anchored on my first,
Misses 1 of my first

This describes the drawing of a letter A ('Shaped like my first') using component letters ('from my first'), like this:

    A
C C
M   M
E     E 
Note it is 'anchored' on two A's, but is missing a character to draw the horizontal line across the middle ('1 of my first').

Catching a loss,
Taking two more,

If we 'catch a loss' - add an L - then the shape becomes a complete A ('my head'):

    A
C C
M L M
E     E 

If it then 'takes two more' in the bottom row, we get 'a triangle instead':
    A
C C
M L M
E L L E 

When filled as so,
My third row's a scheme;
And then, my end,
A French magazine

An MLM strategy (from row 3 of the triangle) is a multi-level marketing scheme. Elle (row 4) is 'a French magazine'.

My first remains quite solitary;
Like I too it has matches nary
My products made can be quite scary;
When defective, effects may vary

Back to a typical riddle! 'A' is the indefinite article for a singular ('solitary', with 'matches nary') object. And in Looney Tunes the ACME products tend to be traps ('can be quite scary') used by Wile E. Coyote for catching his nemesis, Roadrunner. But do they ever work??! No! ('defective, effects may vary')

As for the title:

Not only is ACME a company, but the word 'acme' means 'peak', 'top' or 'best'...

• I've proposed an edit getting some of the missed points, but you were very close
– Avi
Commented Nov 14, 2019 at 1:32
• @Avi Next time please consider giving hints vs. “ghost-solving” the puzzle. It’s generally more satisfying for both setter and solver if the solver completes the solution with hints rather than having it written in for them.
– Rubio
Commented Nov 14, 2019 at 2:43
• @Rubio Thanks, I'll keep that in mind; it's happened to me before, so I thought it was OK. However, apparently it isn't.
– Avi
Commented Nov 14, 2019 at 3:02
• Ok, so I was very much on the right track - thanks @Avi. I do tend to agree with Rubio that it's preferable to be given a nudge towards the finish line when you're close rather than being teleported straight to the end, but I'm sure you'll bear it in mind next time :) (Thanks for the monospace fix too - couldn't suss it on my phone at midnight last night!)
– Stiv
Commented Nov 14, 2019 at 6:19
• @Stiv With regards to the monospace fix, I learnt it from someone on this site - just passing it forward :) With regards to Rubio's comments, I wholeheartedly agree
– Avi
Commented Nov 14, 2019 at 19:12