You can learn a lot from a good letting agent. By checking with an agent, you will have a much better chance of getting a property that will rent fast, not something that will sit vacant - eating up your money.
Even if you manage the job of finding tenants yourself it is always worth asking an agent what sort of properties let best in the area you are looking to invest in.
To be on the safest ground it's best to ask an agent who just does lettings as the answer won't be biased by any need to sell you a property.
Of course, it's also best to find out what will let best before you actually go and buy a property - to avoid being stuck with something that is hard to let.
Unsurprisingly, letting agents say that properties that are easiest to let are those that there is demand for and which are sensibly priced.