What would be the ideal spots to build/start a colony on Mars? I am looking for. . .
- Good location; a particular well placed crater, lots of caves nearby, etc. . .
- Good mix of resources; Ice, iron, what have you. . .
The initial colonies' location will be constrained by geography, geology, economy, and (of course) the technology of the time.
Most forms of ground-to-orbit transport like to work best along the equator. Most forms of spaceport prefer large areas that are broad, reasonably flat, and (preferably) sparsely populated because catastrophic failures might occur from time to time.
Not too far away are the economic activities (like mines or factories) that provide the export cargo. Export cargo, of course, justifies the expense of the colony and of the space freighters that link the colony to the rest of our civilization. There are also non-export industries like food, energy, and space-fuel. These vital economic activities can be in many different terrains and geologies.
Finally, between the spaceport and the economic activity is the colony town. This is a safe distance from the spaceport, but also likely to be in a large, flat areas. It houses transients, services the space industry, warehouse and customs, commerce and market, supporting industry, and much more. It's also a land transport hub, with tendrils radiating out to all those outlying industries.
As a bit of a hand wave, I will make the assumption that the material for a Martian space elevator has been developed, and the colony sets out with a gigantic bobbin holding the coiled up space elevator.
Once arrived at Mars, the bobbin is positioned in synchronous orbit over the Tharsis volcano chain, and the elevator unspooled with one end descending and the opposite end rising into space until it makes touchdown on Pavonis Mons
Space elevator being unspooled towards Mars
You will be on the Martian equator. The volcano can be used as the staging point for aircraft or ligher the air craft to explore the rest of the planet, while you can also drill into the volcano itself to build shelters, mine for minerals and volatile elements and even have the possibility of using volcanic caves, lava tubes or "bubbles" in the rock.
The Tharsis volcanoes also lie near the southern shore of the former Boreal Ocean, so if in the future Mars is terraformed, your space elevator/spaceport is also lying near the future ocean.
Terraformed Mars looking down on the Tharsis region, @ 3000 AD
Since water is indispensable for humans, the colony will constantly be melting ice into water to sustain the human population, so your colony should be near large quantities of ice. Ideally, unfreezing a water source could be the solution. It would be something heavily guarded, since affecting the water source would affect the entire colony.
The colony needs to be close large quantities of coppers, iron and chromium, as they are necessary for construction and would make for a source of profit for the colony.
Finally, your colony needs to be on a location safe enough from the elements of the planet, especially the refineries and construction sites, since these would be essential for the growth of the colony.