I need to write short email that will state something like:
..that will turn a good company into a ... company.
What would be a proper word that I can use?
Hrm, just a few observations on some of the answers here, from an American English speaker:
Many English versions of the word you're looking for were historically used to refer to things that are large in size, and can sometimes cause confusion on whether it's exceptionally good, or exceptionally large. In context the difference is often clear, but when describing a company (which could be made larger) the meaning may not be so clear. Be careful when using words like "tremendous" or even "great" / "grand".
Similarly, many English words come from descriptions of things that are so exceptional that they are better than real life. "Fantastic" can frequently be used to describe something that has an element of fantasy, which it's unlikely that your company would have. "Legendary" and "epic" can suffer from the same. Additionally, due to its (over)use in modern informal speech I think "epic" has lost much of its weight and may also appear informal, as user13267 mentioned.
"Marvelous" and "phenomenal" aren't bad suggestions at all, but CAN have an air of supernatural greatness (i.e. more "great" than something can ever be in real life) similar to the previous bunch. Personally, I think these would probably work in the given context.
"Exemplary" isn't bad, but may sometimes imply something that sets an example for others to follow. This might fit your case, however.
I like Thomas Mario Adams III's suggestion of "outstanding", particularly because this company would literally "stand out" from among other merely "great" companies. It's also term that is very familiar to corporate readers. You may want to add emphasis by saying the company will become "truly outstanding".
You may consider
From the online dictionary, definition 2:
unusually excellent; superior: an exceptional violinist.
In addition to the others I would like to add
Superb, Epic, Legendary
This answer over on English.SE has a good amount of information on intensifiers like this that is worth a read.
I would suggest the word
1. notably or conspicuously unusual; extraordinary: a remarkable change.
2. worthy of notice or attention.