What is the best upgrade to get?

To consider the best upgrades/build, you have to look at both the item itself and its evolution item. For example, The brick’s evolution item increases your max hp, which may not be as useful as the Drill’s evolution item, which increases bullet speed. This means the Drill would actually be a more favorable item, even if the brick has slightly higher DPS/Damage. With this considered, the best items are the Lightning Emitter, which has an EVO that decreases attack cooldown(Increasing damage of all of your weapons that have a cooldown before firing again!), the Durian, (EVO increases range and weapon size), the Drill (EVO increases bullet speed), The high powered lasers (Also evolved by the cooldown upgrade, this leaves 1 slot open so you can get the Hi Power Bullet, increasing your attack by up to 50%! Because the bullet is not a conventional EVO item, using the lightning emitter and lasers allows you to make room for it while evolving all of your items!). Finally, i Recommend the guardian spinners. This weapon has a below average EVO item, but it is amazing for knocking enemies back and keeping swarms away from you. Also very good DPS.

Most of the upgrades are pretty useful, but I particulary like the big rocket that will explode a number of zombies at the same time. Also, the shield related ones are really good too.