What are the easiest good pickaxes to get and how do you get them?

Another option is either the Deathbringer pickaxe (Crimson) or Nightmare pickaxe (corruption). Both of these can mine all pre-hardmode ores, but not hardmode ores.

The Deathbringer pickaxe is the fastest pre-hardmode pick (besides the Bone Pickaxe, although the Deathbringer is much easier to obtain and can mine more blocks) for most common blocks, with the Nightmare pickaxe being only slightly slower.

Both of these pick axes require 12 crimtane/demonite bars and 6 tissue samples/shadow scales. Crimtane/demonite ore can be found in very small veins underground, or farmed from easy bosses such as the Eye of Cthulhu.

Tissue samples and Shadow scales however are only obtainable by defeating the Brain of Cthulhu/Eater of Worlds. Conveniently, these also drop large amounts of crimtane/demonite ore.

These can be summoned by breaking 3 crimson hearts/shadow orbs, found in the chasms of your Crimson/corruption respectively.

However, I recommend that you summon the bosses manually, rather than breaking the Crimson hearts/shadow orbs. This way you can set up an arena for the fight that suits you, without the restrictions of the chasms of your Crimson/corruption. I found that a dirt bridge over the top of the Crimson/corruption works well enough, as long as you have the aid of rocket boots or one of their upgrades.

TL;DR: farm Eye of Cthulhu for crimtane/demonite, defeat Eater of Worlds/Brain of Cthulhu, enjoy one of the fastest pre-hardmode pickaxes!