General suggestions for blocks and alloys
These are just suggestions, not rules -- not yet, anyway. I don't claim to have all the answers, but I do get to test new molds every day, which gives me a heck of a lot of experience with different combinations of blocks and alloys.
Brass molds are often magic with 200 - 270 gr. wheelweight bullets. This seems to be a sweet spot where brass can do no wrong, and yields are very high.
If you are like me and preheat molds by dipping the front corner in the pot, well, don't do this with hi-tin alloys, because if you preheat too long the hi-tin alloy can actually solder to the brass.
Black powder alloys iron is the best material for black powder alloys, hands down. Iron runs hotter than other materials, and black powder alloys like to run hot. Iron expands and warps less than other materials, too. Conversely, brass is my least favorite choice for black powder alloys, because brass is prone to warping at high temperatures. Note that my blocks were not originally designed for black powder alloys. They are too big and take a long time to warm up. If you were going to design a block just for BP alloys, you would make the blocks thinner and smaller so they would run hotter.
Iron molds Except for black powder alloys, iron is not the easiest material to cast with, in fact, it's kind of prone to overheating. Nonetheless, I respect iron because it is more stable than the other materials. It doesn't expand as much when it gets hot, so there are fewer problems with shifting alignment and warping.
Aluminum molds aluminum casts heavy wheelweight bullets better than any other material. You do have to baby aluminum by keeping the sprue plate lubed. Aluminum can warp or shift alignment, not as often as brass, but it happens, especially if the mold gets too hot.
wheelweight is my favorite alloy, hands down. Maybe it's just because I am used to it, but I can almost always get good yields with wheelweight. I think wheelweight has a bad reputation because most other brands of molds have a sprue hole that is too small for good fill with wheelweight, and likewise most bottom pour pots don't flow enough for wheelweight, and because many of us learned to cast from the Lyman manual which reads as if the Lyman company owns a tin mine. Give wheelweight the right equipment and the right pour technique, and it will yield well at modest temperaturess. The day may come when I make molds only for wheelweight.
20:1 Very popular with the BPCR and CAS crowd, but I have no use for it. Like most BP alloys, it has to be cast very hot or else it will wrinkle. I find it easier to cast with wheelweight, and wheelweight is cheaper and about the same hardness as 20:1.
medium hard alloys like WW/lino blends were my favorite when I started casting but now they are my least favorite. As a beginner, I got better fill with medium hard alloys, probably because the molds I was using had itty bitty sprue holes and the bottom pour pots I was using had itty bitty streams. Gradually, I learned to get good yields with WW, and I also learned that the medium hards were highly susceptible to heat shrink, especially with heavy bullets and iron blocks. The only reason I would use a medium hard alloy today would be to avoid the age-softening problem of HTWW.