Have to disagree with LeBreton, I think there are apples that do not make useful contributions to cider and can therefore be called "bad" cider apples. I'd put many of the commonest dessert fruits in this category. But then, I say that many of the widely-sold ciders in America are bad ciders, so maybe I'm not a reliable witness.
Having said that, to answer the OP, I grow and ferment a fairly large number of cider cultivars, and have been experimenting with blends for several years now. My #1 recommendation is actually an old American apple, King David, which (at least in my climate/soil) develops enough tannin to be considered a bittersweet, and its fruit character persists through fermentation better than any other variety I've tried. It actually is too fruity for a single-varietal, IMO, but 30% to 60% of it blended with almost anything else will make a pretty good cider. With Muscat deBernay it can make a GREAT cider.
Some of my best blends have been:
60% King David, 40% Muscat deBernay
40% Muscat deBernay, 30% King David, 30% King of Tompkins
40% Taylor's, 40% Ashmead's Kernel, 20% Cox's Orange Pippin
60% Golden Russet, 40% Yarlington Mill
30% Sweet Coppin, 30% Tale Sweet, 20% Porter's Perfection, 20% Cox's
I'm in a cool-summer climate with lean sandy soil, and those conditions seem to favor tannin development over sugar. If you are in a hot-summer climate you might need less of the sweets (Golden Russet, King of Tompkins, Cox's, Ashmead's, Tale Sweet, Sweet Coppin) and more of the bittersweets (King David, Muscat deBernay, Taylor's, Yarlington Mill). You might also notice I have few sharps, because I don't like acidic ciders and get plenty of acid from the other fruit - again, probably my climate. YMMV.