A lot of people are saying snaffle but despite its simplicity, I disagree.
Before investing in a bit it's important to look at your horse's mouth. Check to see the size of his tongue and how low the palate is. If his palate is low, the nutcracker action of a snaffle will dig into the roof of his mouth and most likely cause pain responses such as head tossing and excessive chomping on the bit. If he has a small narrow tongue, the action of a snaffle will put more pressure on the knife-like bars of his mouth, which is extremely uncomfortable for the horse.
I'd begin with a bit which is more contoured to the horse's mouth, such as a double jointed bit rather than a single jointed snaffle. There are various options out there such as a lozenge bit, french link, roller... They focus more on tongue pressure. My mare has a large tongue and although we ride bitless she rides best (in terms of bits) in a happy mouth double jointed roller.
I'd hate for you to waste your money, so taking a look at your horse's mouth anatomy will really help assist the kind of bit and bit action you're looking for.
I’d recommend a loose ring snaffle. If your horse responds well to it then there is no need to get a stronger one. If they seem to ignore it and run through it then you can go up to a stronger bit. Good luck!
It depends on the discipline. No bit is “harsh” to begin with. It’s simply a piece of metal sitting in the horse’s mouth. What makes a bit “harsh” is the hand at the bit. Inncorrect use of any bit, will make it “harsh”. However, I recommend a D-Ring snaffle. Snaffles are known as training bits. Although snaffles are said to be the softest bit, as mentioned, no bit is harsh until the hand at the bit puts too much force behind it. I hope this helps.
it depends on the horse and what discipline you are training the horse to do, but I typically get to know the horse and then decide, but i usually use the but with the key for them to play with when i train.