There are different ways to measure 'best', aren't there?
An (J)ISA is just a wrapper that avoids tax. Many cash ISAs have a better interest rate than 'normal' savings account and this is what makes them worthwhile - it's pretty unlikely your 10yo is worried about tax at the moment. The other good thing about them is that by their legal structure the child cannot have the money until they are 18, and that may appeal to you.
So, there are two main types:
cash, and
stocks
Best for cash is probably simply dependent on the rate. I think Nationwide is pretty good. (3.25%)
You may also want to consider how their online systems work and how good their customer service is.
Best for investments is a whole nother issue. As well as looking at how their systems work and their customer services, you probably want to think about what it is actually invested in.
You will want to consider levels of risk, diversification, total return, historic returns, charges, total expense ratio, types of scheme management (i.e. active or passive).
Personally I would go for a FTSE 250 index tracker with a solid provider like L&G, or HL. (L&G seem to have an all-share tracker: www.legalandgeneral.com/investments/isas/junior-isa/ )
But you may prefer to manage it yourself, so just open an account and buy individual stocks of your own choosing.
It also depends how much you are putting in, how often etc.