What is a formal word for "go-to"

Solution 1:

I would say that if some [noun] is your go-to, it is your [noun] of choice:

to be preferred

Jet skis are the watercraft of choice for exhilarating beach entertainment.

[Merriam-Webster]

Solution 2:

preferred - 1. To choose or be in the habit of choosing as more desirable or as having more value

http://www.thefreedictionary.com/prefer

Your sentence becomes: "...Jet skis are the preferred watercraft for exhilarating beach entertainment. ..."

Solution 3:

I might use "default" to indicate the normal selection from a list of options.

"Canonical" means most accepted or authoritative but is used more for ideas than for things.

Your jet ski example suggests that you may be thinking of things, so these suggestions may not suit your requirement.

You may be trying too hard; go-to is fairly acceptable. You could also just say why something would be the go-to choice: Jet skis are the ideal choice for....

Solution 4:

If you want a grandiloquent term, perhaps quintessential

Representing the most perfect or typical example of a quality or class

Oxford Dictionaries Online

Solution 5:

Sometimes I hear the usage

de facto/de facto standard

SQL Server is the de facto standard server technology in the .NET development field.

A de facto standard is a custom, convention, product, or system that has achieved a dominant position by public acceptance or market forces (such as early entrance to the market).

https://en.wikipedia.org/wiki/De_facto_standard

It differs from the de jure standard, which is something officially endorsed by a central body.