Entei is a roaming Pokemon, so it can be on any route. Check your Pokedex to see Entei's location; it changes when you go into a new area.
Route 45 is south of Blackthorn City.
Route 38
route 29,49
route 14
You trade it with a girl on route 14 in crystal for a chansey, which can also be found in route 14
route 202 u need to have entei and raiuko in your party
I found Entei at route 38 but it will take a while to find Entei because legendary Pokemon are very very very hard to find.
there is not 1 certain route and you have to of chose squirtle to get entei
Entei will appear if: You chose bulbuasaur as your starter Pokemon and if you beat the elite four obtained the national dex and gave the ruby and sapphire to celio. Once all of this is completed you can find Entei on any route where grass is, you can't find it on the islands. I suggest you go to Route 1 to find it because it usually goes there.
Route 14, however, it is rare.
route 38