Good question. And the answer isn't in all cases absolutely clear from Shakespeare's play.
Desdemona is faithful to Othello. And he should realise it: it's not like she's actually had a night free since they were married to cuckold him with Cassio. The night of their marriage they're all off to Cyprus - Cassio and Desdemona on different boats, and thereafter are apart, and the first night of their arrival in Cyprus is spent splitting up the Cassio/Montano brawl - she's with Othello the whole time. The next night he kills her.
Desdemona's supposed 'infidelity' is basically impossible: testimony to the power of Iago's persuasiveness.
Emilia - we just don't know. Iago seems to think so at one point (though when can we believe anything he says?):
I hate the Moor:
And it is thought abroad, that 'twixt my sheets
He has done my office...
Iago suggests that Othello and Emilia have had an affair. And later on, in the Willow scene, Emilia says to Desdemona,
But I do think it is their husbands' faults
If wives do fall: say that they slack their duties,
And pour our treasures into foreign laps,
Or else break out in peevish jealousies,
Throwing restraint upon us...
There's no evidence anywhere to confirm or deny. But Emilia seems pro-adultery: and Iago suspects her. It can be argued both ways.
Bianca is faithful to Cassio in that she loves him, but unfortunately, she sleeps with other men - for money! Here's Iago on her job as a prostitute:
Now will I question Cassio of Bianca,
A housewife that by selling her desires
Buys herself bread and clothes: it is a creature
That dotes on Cassio; as 'tis the strumpet's plague
To beguile many and be beguiled by one...
She's not really got a husband to be 'faithful' too as such. So - as you can see - Desdemona is the only definitely faithful woman in the play.
Hope it helps!