Is there an informal term for the "best company in an industry"?

I think the term you're looking for is market leader.

A market leader is a company that has the largest market share in an industry, and which can use its dominance to affect the competitive landscape and direction the market takes.

_{Source: Market Leader Definition | Investopedia http://www.investopedia.com/terms/m/market-leader.asp#ixzz42aZkmi6a}

I would go with top dog for this. As defined by dictionary.com

a person, group, or nation that has acquired a position of highest authority.

I would posit that a company can be considered a group, and therefore the term could be applied to one.

Forbes and rt.com support this usage of the term in contexts involving nations or companies.

"IBM used to be the top dog of the IT industry in the past" should be recognised by most native speakers as a reference to their dominance and success.

The leading example in any field is known as the Paragon.

A paragon is an example that is held to be an example of excellence.
The word is used in many fields, industry being just one of them.

This is not one word, but for a common expression, I'd suggest "(the) 800 pound gorilla in/of ... industry."

"IBM is the 800 pound gorilla in the IT-industry."

https://en.wikipedia.org/wiki/800-pound_gorilla

"800-pound gorilla" is an American English expression for a person or organization so powerful that it can act without regard to the rights of others or the law. The phrase is rooted in a joke riddle:

"Where does an 800-lb. gorilla sit?"

The answer:

"Anywhere it wants to."

This highlights the disparity of power between the "800-lb. gorilla" and everything else.

The term can describe a powerful geopolitical and military force, or, in business, a powerful corporate entity that has such a large majority percentage of whatever market they compete within that they can use that strength to crush would-be competitors. (The metaphor includes an inherent bit of hyperbole; the highest weight yet recorded for an actual obese gorilla is 600 lb. (270 kg). The average weight is 400 lb.)

The metaphor has been mixed, on occasion, with the metaphor of the elephant in the room.[1]

Try cream of the crop.

Fig. the best of all. This particular car is the cream of the crop. These three students are very bright. They are the cream of the crop in their class. - The Free Dictionary

It's a little awkward in your sample sentence but here's a slight rewording:

In the IT industry, IBM used to be the cream of the crop.

Since you're looking for colloquial usage, here are some examples from around the web:

• IBM has always been the cream of the crop. - shakarocks
• IBM was the cream of the crop. - Anders Bylund
• IBM was the cream of the crop, until the 75 GXP. - Bozo