Which one is the best attacker??

Tyra With bite and se anytime...
But db drag is still ok whatever charge move you ve got.

Never had the opportunity to use a DP Dragonite and DB Gyarados so this may be a bit biased. If ever, I'd choose (in this order) Snorlax, Dragonite, Gyarados and Vaporeon.

Number one. After that it's Gyarados. Vaporeon and Snorlax are equally useful

For neutral attacks (so no advantages) and whatever doging strategy.
1. Snorlax with L/HB - highest damage before it faints outside of Blissey. But no super effectivenes. Can dodge easely.
2. Vaporeon with WG/HP - only DT Dragonite, BIte Tyra and Lick Snorlax outclass him on damage before fainting. Can dodge easily
3. Dragonites with DB/HB, DB/DP - 2% under HP vape on total damage :). Easy to dodge
4. Gyarados with DB/HP - 11% under Vape.

Type advantages:
Dragonite vs. Vapes
Vape vs Rhydon
Snorlax?
Gya?

Type disadvantages:
Vape vs. Dragonite

Speed:
Dragonite >> Gya > Vape > Snorlax

Dragonite seams to win, whchever you like more from them. And Gya appears to be the clear 4th.

Or more simple, pokebattler.com :), will tell you quite the same: http://pokebattler.com/rankings/attackers/levels/30/defenders/levels/35/strategies/DODGE_SPECIALS/DEFENSE?sort=POWER&view=SINGLE . Play a bit with Dodging types, Snor, Vap and Drag will switch places a bit.

Against what? The damage output changes depending on what you are attacking into, given that SE=STAB in terms of ATK bonus, the differente it's not as prominent, but it's still important enough to make this argument pointless.