Tenn Walker ... round as a barrel. great for bare back. not so great to keep the saddle on straight. Can't mount from the ground, the saddle will slip around with just a bit of pressure in the stirrup. Positive side effect... I've learned to ride very centered.
Cleveland Bay crossbreds. They have good withers, not freakishly high, but tall enough to hold saddle well, along with great width of the shoulders. Add in their very short backs that give them strength, and they are difficult to fit well.
The bonus is that when you find one saddle that fits well, that saddle fits all of our horses well, since they are so similar physically. They are chosen here, for matching each other because they are driven together, need to be similar to work well as matching pairs of horses or a Four in front of a carriage.
I do try each saddle on my horses, never would buy a saddle without trying it on because these horses just are not a "standard" fit, ever.
My 15.2 Westphalian is in an XW freedom tree Black Country, and there was some question whether THAT was wide enough for him. For awhile, I borrowed a saddle that was actually too wide for him (hard to find!), and it belonged to a 14.3 Welsh Cob!
Almost as wide is the 16.3 Trakehner/Dutch ... If he was still hunter fat, he'd be in the XW for sure, but since finding out he is insulin resistant, I keep him pretty trim.
I have an Appendix Bred quarter horse whose if 15.2 and built like the typical bulldog quarter horse. I still struggle to get a good saddle fit on him and know I will have to go custom at some point. My regular quarter horse is the one who looks Appendix bred. I use FQHB on her.