Type I - most variable category in terms of quality. TDK AD tape had a higher than normal coercivity, giving it a rising high end, good SOL, but poor MOL. The best Type I tapes could be excellent in terms of quality, but they all had a 4.5 dB disadvantage in terms of the 120 microsecond playback EQ. Type I is generally the most stable of all the tapes. BASF manufactured the IEC I standard.
Type II - more costly pigments meant less variability in quality. The 70 microsecond EQ gave these tapes a 4.5 dB advantage in terms of lower tape hiss, but that EQ setting sacrificed some--not all--of their SOL advantages over Type I. There were two basic pigments used: chromium dioxide and cobalt-doped iron oxide. Chrome initially had problems with lower sensitivity, MOL, and print-through; but BASF developments in milling and pigment development improved all these low points. Combined with its distinct advantages in low noise floor, low modulation noise, and freedom from magnetostrictive losses in high end over time, chrome's advantages made it the choice for the music industry's best tapes. Double-coated chrome tapes (Chrome Super and Chrome Maxima) had MOL levels equal to that of the better chrome equivalent tapes without a sacrifice in noise levels.
Cobalt-doped tapes have high MOL levels and generally greater sensitivity than chrome tapes (partly due to higher packing densities), and the fact that the playback meters showed higher output gave them a distinct marketing advantage. They did suffer from higher initial noise levels and increasing noise over time ("delta noise") as well as some loss in short wavelength response due to magnetostrictive effects from capstan/pinch roller pressures. Most had very good resistance to print-through except TDK's SA-X tape which was formulated to beat BASF's claim of "world's quietest tape" for its chrome tapes. TDK probably milled the particles to tiny sizes for lower noise, but that would run the risk of too many crystals with paramagnetic properties that would lead to instability. Cobalt-doped tapes were often more prone to rub-off than chrome tapes, which were noted for the clean running properties. BASF made the intial IEC reference tape using a chrome formulation. In the 1990's BASF changed the IEC II standard using a ferric cobalt formulation.
Type III -- a combination of a ferric oxide base and a chrome upper layer for increased high frequency sensitivity and a low noise floor, ferric-chromes had amazing MOL levels at 315 Hz (due to high packing densities) and the same low noise floor of chrome tapes. But that's all. Their mid-range output levels were quite low, and their SOL levels were very poor due to the imbalance of short wavelength flux penetrating below the thin chrome layer and reaching self-erasure in the ferric base. Both BASF and Sony made ferric-chrome tape; Sony made the IEC III reference tape.
Type IV -- a pure metal pigment tape "reintroduced" by 3M with its Metafine formulation (BASF's iron carbonyl from 1934 was actually the first metal pigment), metal tape had enormous levels of output compared to all other formulations. It needed it. The noise levels were also the highest, probably because the particles were so difficult to disperse and orient in manufacturing. The resulting S/N at 315 Hz was often only slightly better than the best Type I or Type II tapes, but the SOL levels of metal tape were substantially better than any others. TDK made the IEC IV reference tape.
