Best Firms for Substation Engineering?

You've got it all backwards.

A better question to ask is, what can I do as a young substation engineer to prepare myself for success as a substation engineer in the future?

You can't start at the top.

Hard work will get you farther than anything else.

"Which firm is the best for a young substation engineer?" is a very subjective question that can only be answered by the person asking the question.

If you are going to be comfortable with spending your entire career with one company the choice is much different than if you are going to use the system to your advantage for opening your own firm one day in the future.

Smaller engineering firms tend to pay less and have fewer benefits however their engineers are exposed to a much broader spectrum of engineering disciplines and they are expected to perform at a much higher output at a much higher quality level which in turn provides learning opportunities one will most likely never get at a large firm.

Most large firms tend to offer specialized services to their clients therefore one must make a choice which field to enter. (Maintenance, Design, or Installation & Startup?)

In my opinion working for a small firm early on in one's career is extremely good for refining skillsets and identifying which way to proceed in the future.

You may well have to experience several different employers and many different work assignments before you identify what makes you happy and satisfied.

My advice is find out what type of substation engineering you would like to do then find a way to refine that particular set of skills and lastly find the employer that pays the most for the skill set you have to offer.

Applying at/with substation construction firms is a good way to get your feet wet and be exposed to all facets of substation engineering requirements.

If you are in the USA some such larger companies are: Bechtel, Jacobs, LDP, Power Engineers.

If you will perform a web search and visit the employer's employment page you will find there are many opportunities however you may have to be willing to relocate and/or travel in the beginning of your career.

All employers and all job assignments yield good experiences and knowledge if one applies themselves.