This thread is not gonna work if you don't clarify what you consider to be bread, what you consider to be a sandwich, and what you consider to be breakfast (what you consider 'best' couldn't be more clear).
Bread is the stuff that you slice, yeah? That whole grain stuff that can be varying degrees of brown (or white, if you want to treat yourself on a sunday), and that's really fresh ideally (as in; not in fucking plastic). That's bread?
A sandwich is two pieces of bread, and in between there's stuff, yeah?
Breakfast is eating after you get up, between 07:00 and 10:00 normally, yeah?
Working from that I'd have to say two slices of brown bread that has been baked anywhere between 2 days and 0 seconds from the time that I eat it.
A bagel? Not a thing that you really slice, as in 'a slice of bread'. You have bagel shops; that's not a thing for bread. You don't have a 'bread shop'. You have a bakery, where they make bread and some other stuff that is not bread, but that's not really a 'shop'; it's a bakery. And you don't eat a bagel at breakfast, unless you're a dick. And a bagel sandwich doesn't really sound right. Not breakfast sandwich bread.
A baguette (any size)? Closer, but not enough to constitute 'bread'. Definitely not if it's white. I understand why some people would call a whole grain baguette 'bread', but that's as far as my sympathy goes. Also, baguettes at breakfast? A regular breakfast?! Are you Mary Antoinette or something? Not breakfast sandwich bread.
A French baguette (any size)? That's the same as a baguette. Not breakfast sandwich bread.
A Kaiser Bun? That's basically the same thing as a small baguette. Not breakfast sandwich bread.
A roll (or whatever British people call them)? That's just air. Not breakfast sandwich bread.
A croissant? Pastry. Not breakfast sandwich bread.
A slice of sourdough bread, or anything fancy and a little more difficult to make like that? It's technically bread, in that you slice it, so I'll give it a pass. But it's not something that you eat for breakfast. It's more of an afternoon/lunch thing, unless you're a dick. So, fuck the pass. Not breakfast sandwich bread.
Anything else (pancakes, waffles, scones, muffins, and whatever the fuck. Fucking biscuits?!)? It isn't bread. It isn't something that you make a sandwich with. It isn't something that you eat at breakfast. Not breakfast sandwich bread.