I'll agree with K Kel here, it depends on your strategy and what your opponent has as well.
Lightning - Good as a small attack craft (is NOT an AA Mount). It is relatively cheap and can take up to 6 Hellstrikes, Bombs, or Skystrikes (which are AA mounted and can re-roll hits against Flyers)
Thunderbolt - Good interceptor (2 TL Autocannons on AA Mount), and can also take Bombs or Hellstrikes (and possibly Skystrikes IIRC)
Vulture - Excellent gunship. Has a massive variety of weapons to choose, from Multi Lasers, to HK Missiles, to a TL Punisher Cannon. It is the most versatile Flyer and can be kitted out for any role, but tends to perform better against ground forces rather than against air units.
Marauder - Massive Bomb payload of 12. Apart from that it is lightly armed with only 2 TL Heavy Bolters and a TL Lascannon.
Marauder Destroyer - So much better than the Marauder. It only has half the payload as the Marauder, but it also has 3 TL Autocannons, a TL Heavy Bolter and a TL Assault Cannon. It can also take 8 Hellstrikes for +80 Points, allowing it to seriously harm your opponent's forces even after it's payload is gone.
In short:
Anti-Air weapons = Lightning or Thunderbolt
Support Gunship = Vulture
Heavy Bomber = Marauder
Heavy Support Bomber = Marauder Destroyer (personally, I always recommend the Destroyer over the Marauder)
Edit: Also, I suggest you buy both Imperial Armour Apocalypse I and II, rather than using the Flyer rules in Imperial Armour Volume I, as the rules greatly change between the copies.
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