Does anyone know what the lead instrument of the orchestra is?

Well, I suppose one would say the first violin. The holder of the first violin chair is called the Concertmaster.

Scroll down a bit, and be prepared for the fact that this site has audio:

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There are multiple lead instruments in an orchestra depending on the composer and the arangement by the conductor some may be more promienent than others at any give time. Also cetain styles and orchestration favor different instruments.

but "Concertmaster. The "first-chair" violinist (to the conductor's immediate left) supervises the tuning of the orchestra and is a vital liaison between the conductor and the orchestra"

The first first violinist is called the Leader in the UK. The first violin is the leader of a string quartet, too, no matter how democraticaly they organise themselves. The music is written that way. It's always the leader who gives up his/her fiddle to the guest soloist when a string breaks - probably because they are reckoned to have the next best instrument in the house... Concert master has a Germanic sound to it - Konzertmeister - and probably is a translation of that.

It used to be a real power struggle between the first violinist and the real "concertmaster" - a precursor of the conductor who would lead performances from the keyboard. Often times the first cellist would share some of the "Kapelmeister" responsibilities (as in the case of Bocherini and Vanhall).

But as the position of the conductor developed into one where the conductor was no longer playing an actual instrument, the first violinist assumed the position of the most powerful instrumentalist in the orchestra. Concertmasters today deal with a wide range of responsibilities, and have to know how to conduct in case the conductor is temporarily unavailable at a rehearsal.

Every instrumental group (or section) has a principal (or soloist) who is generally responsible for leading the group and playing solos. The violins are divided into two groups, first violin and second violin, and therefore have two principals. The principal first violin is called the concertmaster (or leader) and is considered the leader of not only the string section, but of the entire orchestra, subordinate only to the conductor.

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