What are the easiest wings to obtain?

You cannot obtain wings before hardmode (legitimately anyway). All of them require Souls of Flight from Wyverns, except the Leaf Wings and the Jetpack, which can be bought from NPCs, but only in Hardmode.

The easiest depends on your situation. Crafted wings:

• Angel/Demon Wings
  • Soul of Night/Light x25
  • Soul of Flight x20
• Fairy Wings
  • Soul of Flight x20
  • Pixie Dust x100 (really easy to get from Hallow, guaranteed drop)
• Harpy Wings
  • Soul of Flight x20
  • Giant Harpy Feather (might drop while killing the Wyvern for Souls of Flight)

From NPCs:

• Jetpack
  • 40 Gold, bought from Steampunker during a waxing moon (NPC requires defeating one Hardmode Mechanical boss to spawn)
• Leaf Wings
  • 1 Platinum, bought from Witch Doctor at night (must be living in a jungle biome in Hardmode)
• Fin Wings
  • 1% chance reward after 10th quest from the Angler, but only in Hardmode

The Terraria wiki has a full list of Wings. In general, the better the wings, the more difficult it is to find the materials to craft the wings.

No, none can be made pre-hardmode.

The easiest to obtain depends on what you consider easy.

• Leaf Wings can be bought from the Witch Doctor at night after you move him into the Jungle in hardmode.
  • 1 platinum (100 gold), but require no special materials
• Angel/Demon Wings use the most common materials. These are easier if you can't get the money for Leaf Wings
  • 25 Souls of Light/Night
  • 25 Souls of Flight
  • 10 Feathers
• While trying to get Feathers for the Angel/Demon Wings, you may find the rare drop Giant Harpy Feather. If you happen to find this, you can make Harpy Wings for fewer souls.
  • 1 Giant Harpy Feather
  • 20 Souls of Flight

So basically, you're choosing between spending your hard earned money, and fighting hardmode enemies. If you can't kill hardmode enemies, go for the Leaf Wings. If you can, go for the Angel/Demon Wings and hope you get the materials for Harpy Wings.