I'm a 34HH and find layering up these tops from Next is my best option for daywear. They do quite a few colours and are on a multi-buy offer as well, so very affordable.
I have them in white, navy, black, prune (darkish purple), grey, blue and pink.
I know Next aren't all that popular on here, but for basics, rather than distinctive styles or trends I think they're fine.
I was concerned that the neckline might be a bit too high when I first ordered them but it's actually just right for me. They also have the advantage of coming in petite, regular and tall, which is great for someone like me who is longer bodied and sick of tops exposing my lower back when I bend.
I find slash and boat necks don't flatter me as they tend to create a sort of shelf effect where the fabric sits across my boobs. It just doesn't look right on me.
I do however have a couple of cowl necks, which is a surprise to me, as I've always steered away from them in the past, but these particular ones seem to flatter for some reason.