On re-reading your question, I'm not quite sure what you wish to use the cores for.
If you wish to construct inductors for use in tuned circuits then Iron Powder cores of Mix 2 (Dark Red coating) for Low HF frequencies and Mix 6 (Yellow coating) for the Higher HF and lower VHF frequencies are generally the norm. As they introduce a very small resistive component and so are suitable for the construction of high Q circuits. Type 61 Mix ferrite can also be used for this purpose. If a greater amount of inductance is required for a given number of turns. However none of these materials are particularly suitable for the construction of 'current' baluns. As they do not provide a sufficiently high enough value of resistive choking impedance.
Ferrite core materials are much better suited for the construction of baluns. However the material needs to be chosen to best suit the required operating frequency. Which is where Steve, G3TXQ's charts come in handy.
As a 'finger in the air' rule I'd say:-
Type 73, 75 & 77 Materials are best for the LF bands 500KHz, 1.8MHz & 3.5MHz
Type 31 for 3.5MHz, 7.1MHz & 10MHz
Type 43 & 52 for 7.1MHz through to 24MHz
Type 61 for 14.0MHz through to 54MHz
However this is only for guidance. The actual choice really depends upon the use it is being put to, and the number of turns required for a given value of common mode choking impedance. There are lots of factors that have to be taken into account, in order to produce a satisfactory design. There are a LOTS of poor and misleading construction articles for Baluns and Ununs on the web. Particularly those using powdered Iron cores.
Also beware of unmarked or unknown ring cores !!!!
Many of the types being sold (particularly on the internet or at ham table top sales), were intended for use as storage inductors in switched mode power supplies. They are not at all suited for the construction of Baluns or Ununs.
Martin - G8JNJwww.g8jnj.webs.com