Context. Quasars radiating at extreme Eddington ratios (hereafter xA quasars) are likely a prime mover of galactic evolution and have been hailed as potential distance indicators. Their properties are still scarcely known. Aims. We aim to test the effectiveness of the selection criteria defined on the “4D Eigenvector 1” (4DE1) for identifying xA sources. We provide a quantitative description of their rest-frame UV spectra (1300–2200 Å) in the redshift range 2 ≲ z ≲ 2.9, with a focus on major emission features. Methods. Nineteen extreme quasar candidates were identified using 4DE1 selection criteria applied to SDSS spectra: Al IIIλ1860Si IIIλ1892 ≳0.5 and C IIIλ1909/Si IIIλ1892 ≲1. The emission line spectra was studied using multicomponent fits of deep spectroscopic observations (S/N ≳ 40 − 50; spectral resolution ≈250 km s⁻¹) obtained with the OSIRIS at Gran Telescopio Canarias (GTC). Results. GTC spectra confirm that almost all of these quasars are xA sources with very similar properties. We provide spectrophotometric and line profile measurements for the Si IVλ1397+O IV, C IVλ1549+He IIλ1640, and the 1900 Å blend. This last feature is found to be predominantly composed of Al IIIλ1860, Si IIIλ1892 and Fe III emission features, with weak C IIIλ1909. The spectra can be characterized as very low ionization (ionization parameter, logU ∼ −3), a condition that explains the significant Fe III emission observed in the spectra. xA quasars show extreme properties in terms of C IVλ1549 equivalent width and blueshift amplitudes. C IVλ1549 shows low equivalent width, with a median value of 15 Å (≲30 Å for the most sources), and high or extreme blueshift amplitudes (–5000 ≲ c(1/2) ≲ –1000 km s⁻¹). Weak-lined quasars appear as extreme xA quasars and not as an independent class. The C IVλ1549 high amplitude blueshifts coexists in all cases save one with symmetric and narrower Al IIIλ1860 and Si IIIλ1892 profiles. Estimates of the Eddington ratio using the Al IIIλ1860 FWHM as a virial broadening estimator are consistent with the ones of a previous xA sample. Conclusions. xA quasars show distinguishing properties that make them easily identifiable in large surveys and potential “standard candles” for cosmological applications. It is now feasible to assemble large samples of xA quasars from the latest data releases of the SDSS. We provide evidence that Al IIIλ1860 could be associated with a low-ionization virialized subsystem, supporting previous suggestions that Al III is a reliable virial broadening estimator.