Don't really NEED anything in an embark, but what I look for (to the point of OCD actually...) generally is
multiple biomes. The more the better. 3 is probably the minimum I consider now, 4 is good, 5 better and any more in a 3x3 embark is pure gold. In 43.xx I have taken to embarking where the temperate zone meets the tropical zone, once again pretty much to the point of compulsion.
Other things I strongly look for are an above ground water source (usually brook/stream/river), high savagery in as many biomes as possible, all civilizations as neighbours - bonus if I can be nearest to dark fortress(es).
Metals are good, more is better, sand and/or clay are good, soil a decent poorer relative. Flux is good... for building mainly, and I still like my trees (definitely a persistent hangover since 34.xx
).
Unusual geographic features are great for variety and sometimes override above considerations - canyons, river confluences, 'pre-made' animal enclosures, etc.
So while we're at it, any suggestions for fun (or !FUN!) alternatives to try?
If you haven't tried it an embark without metal but with sand and trees can be fun/!FUN! - think massive glassworks... Can forgo the trees if prepared to quickly access magma.
Not in the realm of geography but fun/!FUN! can be had by trying a more 'peaceful' challenge - a vegan/vegetarian fort. (Strict vegans will need to forgo the crossbow.
) Beware of moods!