Strapless bras are generally useless. I've lost track of the number I've bought and discarded soon after because they simply don't work.
The first issue is that generally, in order to provide support that isn't there because of the lack of straps, they are very substantial. This means that a low neckline can't be accommodated.
Some strapless bras are constructed of such heavyweight materials, in an attempt to provide support, that they don't mould to the body and the wearer spends all day hoiking the bra back in place.
Then there are the multiway bras which are the worst of all strapless bras. As a bra worn with straps they are overly bulky and cumbersome and as a strapless bra they usually lack shape and support.
Wonderbra has the answer and they call it the Ultimate Strapless Bra.
I've used it, more than once, and it is, quite simply, brilliant and well worth the money (about £34).
The first this that impressed me was its ability to stay in place. All day. Once it's on, that's it until you, or someone else, decides it needs to come off. And it's as comfortable as any other bra but so much better than any other strapless bra I have ever bought.
The second thing is that it can be worn with low top. It sits quite low which means that the dip at the cleavage point is quite low.
The last thing is either a plus point or a negative depending upon your point of view. It enhances cleavage. I don't need or want that and I think it makes me look like a matron but there are plenty of cleavage enhancing bras on the market which tells me there's a demand. As far as I'm concerned it's a disadvantage I'm happy to live with and I know others will see it as a bonus.