Ohh ok. If he is currently bitless, maybe you should start off with a mild bit and then see how he goes. Not sure exactly what your situation is but start with a simple bit like a snaffle and then if you think you need a stronger bit maybe go into a leverage bit like a pelham or a gag.
Thanks guys. I have been recommended a stronger bit because my horse is usually good mannered but in last few weeks he's just been tugging. My trainer thought this is because he was currently bitless sothanks.
I agree with @alixdubineau. Changing the bit isn't the best solution. Do the exercises she recommended you to do and check with a vet as well.
To actually go into the bit part, I say it depends. You have to make sure the horse is comfortable and likes the bit, or else he or she will push against it(or show other signs of discomfort) and not make the situation any better.
I say start off with a simple bit like a loose ring snaffle or a D ring snaffle. Then if that doesn't work, slowly move up with the "harshness". Try a rubber pelham snaffle, gag, or waterford. Trial and error is the key to finding the perfect bit for a horse, you might get it after the first try or it might take weeks or months to find one.
If you have a trainer, I recommend you talk to him or her about it, they'll give their knowledge on bits and what they think you should try. Do your own research as well. See what your horse might like best and what each bit does.
First of all you need to know that changing the bit is not the solution for strong horse that pulls (i guess that's what you meant by "a strong horse").
You need to work a lot on circles to put the weight on his hindquarters, do a lot of transitions, counter-bending, rein back, play with your fingers to relax him, put a lot of legs and try to pull the neck up.
Your hands need to be light, don't pull with him (you'll lose, he has more strength than you), work with loose reins at the beginning so that he doesn't have a support to pull and don't forget not to pull back! you pull then release reins.
Maybe you should see a vet to check her teeth to be sure there is no problems.
Finally, to answer your question, you can work with a pessoa (4 ring gag), baucher bit or elevator bit but you need a very light hand with them and it only covers the problem, you need to work the dressage of your horse for him to be more light on his shoulders.