How about no one cares about Dratini v Bagon? No one battles with first-stage evolutions.
But since you ask, let's take a look. Dratini and Bagon have equal speed. Let's just say Dratini attacks first. You can assume he'll be EV'd in Spd and Atk, as would Bagon. Dratini has only 64 base Atk to Bagon's 75, and it has 45 base Def to Bagon's 60. So even if Dratini goes first, it won't really hurt Bagon too much, and Bagon would likely OHKO Dratini with one of pretty much any physical dragon attack.
So, in short, Bagon would beat Dratini.