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There are probably lots of ways to express this, but it would depend on the context.

"Fallback" (n), or "Backup" are likely easy one word substitutes.

English tends to be less expressive via single nouns/verbs/adjectives, so if you went for a phrase, we may hear "If all else fails, there's always... x".

In fact, words like "dependable" and "reliable" already (within certain contexts) euphemistically express the meaning that it wasn't the best/first choice.

As in "What's he like as a candidate?", "Oh - he's dependable". But that's sort of more in a negative context.

A "Jack of all trades" can be relied on to do many different things. Most Americans will automatically fill in "but master of none", meaning that he does not do an excellent job in any of those things.

In basketball and other sports, a "role player" can be relied on to do his job consistently well, but not at the level of a "star" or "superstar".

"Solid", "steady", and "reliable" are adjectives. If used without other adjectives, the implication is that the person or thing's reliability is its most important feature. Most things that are perfect for a particular task have other adjectives that are more likely to be used if they are applicable.

Something can be a standby when it is ready for use. It is reliable, but if it were the best, it would actually be in use. Quite often it was in use but replaced, but is still good for the job, for example a kettle that has seen better days but still works well.

The Oxford Dictionary has

1.1 count noun A person or thing ready to be deployed immediately, especially if needed as backup in an emergency.

with examples

The tugboat is truly multi purpose, as it can lead oil tankers into port, repair petroleum pipes in the sea and act as a standby rescue boat.

Soup is a great standby, and we Scots are the best soup-makers of all.

The Cambridge Dictionary has

standby noun something that is always ready for use, especially if a
regular one fails.

with examples

Board games are a good standby to keep the children amused if the weather is bad.

There are standby generators but these usually only have to work for a few hours a year during power cuts.

The word "Journeyman" is often used to describe an individual who is reliable but not the best in their chosen profession.

From Meriam Webster:

An experienced reliable worker, athlete, or performer especially as
distinguished from one who is brilliant or colorful

I'd say "competent"

...acceptable and satisfactory, though not outstanding.
"she spoke quite competent French"

Journeyman as Matt Coubrough suggested, for a person. If you need a word that works with things, try stolid.

From Oxford: Calm, dependable, and showing little emotion or animation.

I think you're looking for "Safe" as in "The safe choice" Definition 2 here calls it derogatory, meaning 'Cautious and Unenterprising' but still 'Safe'.

The adjective trusty is a good word to describe something that maybe is not necessarily the best of its kind, but you know is reliable because you've used it for so long that you can rely on it:

Having served for a long time and regarded as reliable or faithful

Example sentence (from the Cambridge Dictionary):

I did the entire three hundred miles on my own—just me and my trusty bike.

I think the word could be "trustworthy", always reliable but doesn't matter on being the best or not.