Biggar has proven over the past year or more to be up there with the best NH fly halves, namely Sexton, who's his only real rival. He's had a couple of quieter than usual games these last two weeks, but he hasn't been particularly poor. Sexton, bar last week has been worse since the WC, so Biggar has probably earned the right to be considered the form 10 in the NH.
Sam Davies hasn't quite earned the right to be considered better than Farrell, I agree. He's had some really impressive displays this season, but so has Farrell. Farrell is playing as part of a superb Saracens team, a fact that most English posters on this forum attribute to him appearing better than he is. Farrell has a few too many weaknesses for my liking, but for the time being he's the more proven performer. Davies is improving quickly, and I think it's only a matter of time till he features for wales.