You mentioned this is Formal Writing you're adding this sentence to, so I would use the in front of the word most. Worth mentioning however, non preferably, the word most is able to stand on its own in this sentence. In addition, I would omit the needless word "That", and replace it with the more appropriate comma after the word "Is". So it reads; "What fascinates me the most about the textile industry is, it drives the economy of many third world countries.". "The most" is also preferred as it is the superlative of the action verb "drives".