Bagon: B Tier
Availability: Fairly early in route 8 before the second gym at level 14, evolves to Shelgon in level 30 and to the poweful Salamance at level 50.
Stats:Bagon....well pretty sucks actually, Shelgon it's more viable and can be pretty sturdy on the physical side paired with Eviolite,but is slow and Salamance...is awesome and can run a mixed set very efectively.
Typing:Dragon is a good type overall,their weakness are not so common...just the Creepy Tale girls and some random pokemon ranger or ace trainer with ice moves can cause trouble, but it resist Water,Fire,Grasss and Electric,wich is good, now Dragon/Flying makes it better,now can resist fighting attacks and got inmunity to Ground moves.
Movepool: Bagon and Shelgon early level up moves are pretty shallow,the most notable moves are Dragonbreath,Zen Headbutt and Crunch and for early coverage and try to make some good damage in TM you got Rock Tomb, Shadow Claw,Aerial Ace,Rock Smash and Hone Claws for setting up, they can learn other useful moves like Flamethower,Dragon Claw and Rock Slide, but by the time you get them you should have a Salamence or very close to it. For Salamance, it learns Fly when evolves in level 50, and gets Stone Edge, Earthquake and a better Special Attack to use Flamethower and Fire Blast.
Major Battles:
Grant- No, Amaura will just kill you and Tyrunt resist your attacks at least you got Dragonbreath or Shelgon.
Korrina- If you already got a Shelgon, equip the Eviolite and use Hone Claws to sweep her with Aerial Ace. But Bagon can't resist very well the hits.
Ramos- Jumpluff and Weepinbell can't do very much against you,just Gogoat can cause you trouble.
Clemont- You can easily take care Emolga, but Magneton and Heliolisk have a very good Special Attack so be wary of that.
Valerie- Nothing to do here.
Olympia- You should have a Salamence at this point, if you have it, just use Crunch and see how everything dies, but if you don't Shelgon can be murdered by Psychic easily.
Wulfric- With a mixed set with Fire Blast and Rock Slide you got a chance if you manages to OHKO his pokemon with the appropiate move...otherwise you're dead.
Shauna- Kills easily Delcatty and her starter,be aware of Goodra.
Tierno- It can't be troublesome...you got the type advantage.
Trevor- Be careful with Florges it has Moonblast.
Calem/Serena- At first they will give you trouble,but when you get Mence you can handle their teams very well.
Lysandre- Salamence resist Mienshao HJK and OHKOes with Fly,handles Honchkrow and Pyroar pretty good, but Gyarados has Outrage so it's not very good idea. If you're stuck with a Shelgon at this point, it's going to be dead weight.
Malva- You got Earthquake...so she's dead, RSlide for Talonflame.
Siebold- Not so good, every member of his team has a super-effective move againts Mence.
Wikstrom- With Earthquake and Flamethower/ Fire Blast you got his team covered.
Drasna- You may be able to OHKO her team with Dragon Claw, but it's risky that they kill you with Dragon Moves too.
Additional Comments: Level up Bagon can be troublesome it dies quickly and you have to reach level 30 to get a more decent Shelgon, Shelgon is more workable is capable of tank hits and hits harder, but even with Eviolite Special Attacks will let him down and Salamence...it's awesome, a very good sweeper with amazing coverage and with Intimidate it can tank some Physical super-effective hits very well. The major flaw is that this dragon family doesn't get Dragon Dance and learn Dragon Claw(Their best STAB) very late. But Salamence has very good stats and a wide movepool (at least for the E4) so you got a very good and dangerous pokemon.