As I discuss in this question, by far the most popular school of Hindu philosophy is the Vedanta school, which derives its tenets from the doctrines laid out in the Brahma Sutras, a work by Vyasa which summarizes and systematizes the philosophical teachings laid out in the Upanishads. It's not a verse-by-verse commentary of each Upanishad, but rather an explanation of many different Upanishads. Insofar it's the defining text of the Vedanta school, it's doctrines are followed by the vast majority of Hindus. You can read the Brahma Sutras here; it consists of concise sayings that are often hard to decipher if you're not an expert on the Upanishads, so it's usually read with the assistance of a commentary. The two most popular commentaries on the Brahma Sutras are the Brahma Sutra Bhashya of Adi Shankaracharya and the Sri Bhashya of Ramanujacharya, which you can read here and here respectively.
As far as what commentaries on the Bhagavad Gita and the Upanishads are widely followed, there are many different philsophies within the Vedanta school. As I discuss in this answer, the key difference between these philosophies is the relationship between Jivatma, the individual soul, and Paramatma, the divine soul or supreme soul. According to Adi Shankaracharya's Advaita philosophy they're identical, according to Madhvacharya's Dvaita philosophy they're totally separate, and according to Ramanujacharya's Vishishtadvaita philosophy Jivatmas are just small parts of the greater Paramarma. Philosophers from each of these traditions have written commentaries. Out of these philosophies, Advaita Vedanta is currently the most popular due to the work of Vivekananda and others, so Adi Shankaracharya's commentaries would be the most popular. But Ramanujacharya's commentaries would be a close second. In any case, you can read the four main Vaishnava commentaries (including Ramanujacharya's) on the Bhagavad Gita here.
As far as where you can read the Vedas and Upanishads in English, as I discuss in this answer the site sacred-texts.org is a good source. Here's the Rig Veda Samhita, the Krishna Version of the Yajur Veda Samhita, the Shukla version of the Yajur Veda Samhita, the Sama Veda Samhita, the Atharvana Veda Samhita, the Shatapatha Brahmana of the Yajur Veda, some major Upanishads from various Vedas, and some minor Upanishads.