Update Spring 2003:
Sorry for bumping up an old thread, but the selection of blank cassette tapes has really dwindled down. Maxell Corp. Of America now distributes only the XLII (Type II) and the UR (Type I) tapes. SonyStyle USA now only has the CD-IT (Type II) and the HF (Type I) tapes. TDK USA still sells the MA (Type IV), SA-X (Type II), SA (Type II), CD Power (Type II), CD Bass (Type I) and D (Type I) tapes for the "consumer" market - but most stores that I've been to now stock only the D tapes; the others are typically sold in 10-packs directly from TDK. There are also three models of TDK tapes for the "professional" audio market, and sold through pro-audio dealers: SM-X (Type II), SM (Type II) and AM (Type I). The formulas for those three are like the SA-X, SA and D, respectively.
As for which tape type is superior, I agree that Type IV is the best of the current tape types - but only if you have a deck that can handle it. But Type II is NOT superior to Type I in every aspect: Type II tapes do have less noise than Type I tapes - but they sacrifice the high-frequency headroom, and sometimes overall headroom, in order to achieve such low noise (in fact, some "true" Type II tapes distort at input levels even at or slightly below 0dB VU). Type I tapes can generally accept higher input volumes than Type II tapes - but oh, the hiss!
And I do agree with the decline of the quality control of TDK's audio cassette tapes in general: Except for the budget-priced D tapes (which remained fairly consistent - and relatively high compared to that of most other Type I tapes), the TDK audio Compact Cassette line had been suffering with erratic mechanics and excessive dropouts in recent years.
That leaves hardly any really good choices in cassettes today. If you
must buy cassette tapes today, the safe (read: decent) choices are the Sony line, and the aforementioned TDK D. (I don't trust Maxell these days, because their XLII barely qualifies as a Type II tape, and their UR doesn't perform as well as the Sony or TDK Type I tapes, IMHO.)
And fuggittabout RadioShack as a source of decent audio cassette tapes: They sell only three models of tapes: HC (Type II) and XR (Type I), both made by Maxell, and the LN (Type I), made by some third-tier company in China. Expect the HC and XR tapes to perform similarly to the currently available Maxell-brand tapes.