Whats indicators show "this brand is successful"?

loyal customers - .i.e - apple

customer repurchase behavior, customers delight a step ahead after satisfaction and customer trust on the brand.

Repeat orders & Customer/Consumer demond

Being the first choice brand in the mind of the largest number of audience in the market, is the main KPI for a successful brand.
Though measuring a brand equity has other KPIs such as sales volume, loyalty, referral, advocacy ...etc Pampers has the first choice in the mind of the consumers, though in sales volume you might find that BabyJoy has a larger market share and loyalty.

Several things come to mind:- 1.
Does the brand have engaged active passionate advocates/ambassadors who promote and recommend the brand wholeheartedly 2.
Is the brand a leader in market share with an edge over the competitors in its unique space 3.
Does the brand have high unprompted brand recognition with positive brand associations 4.
Does the consumer recognize and believe the brand's USPs and tagline 5.
Is there a high level of customer satisfaction and engagement with the product 6.
Is repurchase behavior encouraging 7.
Are customers loyal to the brand (what is the demand elasticity)

Whereas consumer retention is based primarily on effective marketing, successful brands need to fulfill below mentioned criteria to be considered successful: 1.
Consecutive upward growth (financilay) 2.
Visible increase in market share as compared to competitors 3.
Noticeable brand preference by consumers (when the customer chooses your brand from amongst all other options)

Reaching the level of brand insistance, where the customer only wants your product.

when brands become generic we can say This Brand is successful.
For example in India people ask to get a Xerox copy of document instead of saying get a photocopy of the document .
same is the case with detergent powder SURF.
Most of the people ask provide me a surf of XYZ brand.
So, instead of asking for photocopy & detergent powder people ask for Xerox & Surf respectively.