Even though we provide the tutorials and map charts it seems to be hard for noobs to determine what to choose.
I will now try to explain the right way:

1. The best Navcore is always what Home offers as latest official update!
All other Navcores can work but there are no guarantees!
2. We always provide the latest maps in our red stickies, so instead of asking simply check the dates of the postings in the map thread.
Map versions always go up by 5 in the first digits. 895.xxx, 900.xxx, 905.xxxx and so on - every 3 months a new round....
Some maps increase only by one like 901.xxxx, this indicates the map was updated by TT due to major errors.
3. For a downloaded map to work the Navcore needs to be patched and the map activated - you can confirm the later by checking for a .dct file inside the map folder that corresponds to the map name.


With that out of the way, how do you choose the right map?
You need to know the Navcore version number, you can check the info in the menu for that or the contents of the file ttgo.bif.
You also need to know what coverage you need. There are single country maps and especially for Europe the different versions that cover parts of the continent.
To check what map version is current and the coverage as well, you should use the TT website and visit the map shop.
Don't worry, you don't have to buy a map, before you reach that point TT shows you what map is available for your device and the current version number of the map.
With that knowledge you check our map thread.
Scroll down to the map you need and check if there is a meta code posted with the download link - without the meta code the map is not usable!!
If there is no code you either need to wait or use a different map - check the map chart what map version is suited for your Navcore.
In case your version (of the Navcore) is not in the chart the next lower number in the chart tells you what you need.

So much for the maps used on official Navcore versions, now how to use the latest full featured maps with Navcore versions higher than your offical Navcore number.
As said at the beginning, there is no guarantee an onoffical Navcore will work as expected, but we do our best to make the impossible possible ;)
One word in advance: The latest available Navcore version is not always the best the choice!
Devices considered to be compatible with basically any Navcore have the following features:
Screen resolution of 480x272
64mb of Ram
Bluetooth
SD slot
FM transmitter

All other devices have to accept certain limitations.

I already said that a Navcore needs to be patched if you use our maps,
for unofficial Navcores it also means you need to use modified files.
I provide Navcores that are already patched suited for most devices, you can also use the so called SE versions, but they need to be manually adjusted, check the Switchfiles thread in the tutorials.
Those unofficial Navcores should be selected so the suit your needs.
This means selecting an older version can sometimes be of benefit, especially for old 32mb devices.
Be aware that from NC 9.02xx on all map features are supported, so if you already use a Navcore version in that range there is no real need to go higher.
The main difference between the various V9 Navcores is the official support for devices and of course bug bixes (for versions in the same range).
There are some other minor things like "improved route calculating speeds", changes for speed cam handling (especially for France) but they are more or less cosmetic.
If you want to use V9 Navcores on an oldie be aware that sometimes you have to use a different Navcore in case the one you selected won't work at all.
This is a known problem and must be accepted as we not always get what we want in life ;)

Addon: problems after updating
If you follow the above you should not get any problems ;)
But I try to list the most common causes:
You did not use the latest version of FastActivate - always double check if the version you have really is current!
You downloaded a map with no available meta code - check for the meta code before you download a map, especially if you select a map with installer!!
Your Navcore is not patched - if you can confirm FastActivate created a .dct file for your map and you also used it to patch your Navcore, it is possible the patching did go wrong.
In that case try the patching again, if necessary use a different tool, best choice: original yDG patcher for DOS.

If in doubt, before you post for help, do a clean install of map and Navcore.
That means installing all from scratch on an empty device - make a backup using Windows Explorer first, confirm hidden and system files are shown.