General Antenna Info
Antennas are the life blood of any radio system. A bad antenna will ruin any chance you have to get a good enough connection to talk over. That said, there are compromises that can sometimes be made to choose one antenna over another.
For all of these antennas, note that you will want to mount them vertically with the antenna elements pointing up and down like a flag pole. Having them tilted or sideways will result in extremely poor transmit and receive ability.
Similarly, where you place them matters. You want them to be as high as possible and not blocked by metal or thick materials like wood or concrete.
Whip Antennas
Whip antennas are an omni-directional antenna and often the best choice for the vast majority of cases.
"Omni-directional" simply means that they can transmit and receive nearly equally in all directions which makes them great for any radio you are moving around with you.
These will be about 6 inches long and are usually a springy material that can bend. You will sometimes find "whips" that are enclosed in a plastic shell. These will likely work just fine, but it is easier to get them mixed up with other antennas because you can't see through the plastic
A good whip might be this one which will work for most radios with an SMA connection.
Rubber Duck
A "rubber duck" antenna is another omni-directional antenna, but is made up of a piece of wire coiled into a spring shape into a nub or puck.
These can be great if you want a compact radio, but the ability to send and receive distance will be reduced.
While not ideal, something like this will work in many cases.
Built In
For some systems, you need to rely on built-in antennas. Like the rubber duck antennas, these aren't typically ideal, but can work if distance is not a concern.
If possible for your radio and use case, it is highly advised to replace these with a whip antenna.
Directional
Directional antennas can receive or transmit from one direction really well but very poorly in others.
These are mainly used if you need to connect two static (not moving) nodes very far away to join meshes together or just get a connection. Because they only send/receive in one direction, these will need to be mounted to a fixed surface and time spent getting the direction dialed in.
The most common directional antennas are going to be yagi antennas like this. The linked antenna hasn't been directly tested, so buy with your own caution.