Here's a little crash course.
The first thing to keep in mind: medallions, flakes, plugs, cakes, and especially ropes are stronger than pipe tobacco in other forms. Pressed tobacco's are typically the domain of the advanced smoker who wants maximum flavor and quality.
Start with blends/brands/styles you already know you like and then see if there is something similar in a flake or broken flake version.
- English or Balkans are a good place to begin; some good examples include: Bengal Slices, Quiet Nights, Sextant, and Black Frigate (a cake).
- VaPers. Traditionally these come in medallion form (although there are plenty of flake versions too). Good examples include: Escudo, Davidoff Flake Medallions, Luxury Bullseye Flake, Three Nuns, St. James Flake
- Straight (or mostly straight) Virginias. Some good examples: Full Virginia Flake, Best Brown Flake, Orlik's Golden Sliced, Reiner Long Golden Flake (they're not kidding when they say "long")
- Ropes & twists are pretty much their own world and are geared for heroic levels of strength and flavor, combined with a surreal nicotine kick. The Gawith ropes are the standard here (but there's not a lot of competition). Definitely wait on exploring ropes until you've become more experienced with the other pressed blends. Their tongue-destroying reputation is well earned.
There are tons of options and lots of areas to explore. Have fun!