Why did G-d specifically choose to punish the Egyptians with the plagues of blood, frogs, lice, wild animals, pestilence, boils, locusts, hail, darkness, and the death of all firstborns?
In my research over this Pesach, I have found some reasons why G-d chose the Ten Plagues specifically. Many of the answers come from the same source.
The answer is middah k'neged middah (roughly translated as "karma"). Each of the plagues was punishment for something the Egyptians did to the Israelites.
The Egyptians closed all the mikva'ot and did not allow the Israelite women to purify themselves from the niddah blood. Therefore, all their water was turned into blood. (Midrash Tanchuma, Va'eira 14)
Since the Egyptians worshipped the Nile as a god, it was struck and turned into blood in order to show the Egyptians G-d's might. (Midrash Tanchuma, Va'eira 13)
The Egyptians spilled the blood of the baby Israelite boys and threw the bodies into the Nile. Therefore, the Nile was turned into blood. (Abarbanel on Exodus 7:14)
The Egyptians forced the Israelites to retrieve different reptiles and insects in order to torment them. Therefore, the frogs tormented the Egyptians. (Tanna D'vei Eliyahu Rabbah, Ch. 7)
When Israelite baby boys were born, the women had to keep them from crying or making noise, in order that they not be caught and the babies executed. Therefore, the Egyptians were tormented with the noise of the frogs. (Haggadat Zevach Pesach, p. 112)
The Egyptians forced the Israelites to sweep the dust from their streets and shops. Therefore, all of the dust became lice. (Shemot Rabbah 10:7; Midrash Tanchuma, Va'eira 14)
The Israelites were forced to make bricks from the mud from the ground, so all the earth turned into lice. (Abarbanel on Exodus 7:14)
The Egyptians forced the Israelites to hunt in the wilderness for wild beasts for the Egyptian's circuses. (Tanna D'vei Eliyahu Rabbah, Ch. 7)
Israelite children were also forced to work and were shown no mercy by the Egyptians. Therefore, the Egyptian children were harmed by the wild animals. (Abarbanel on Exodus 7:14)
Even the sheep of the Egyptians attacked them, because the Israelites were sent in the wilderness as shepherds. (Haggadat Zevach Pesach, p. 112)
The Egyptians told the Israelites to go into the wilderness and herd their animals, so they were punished with pestilence. (Midrash Tanchuma, Va'eira 14)
The Israelites were forced to pull plows in order that the Egyptian animals not be overworked. Therefore, the animals died. (Haggadat Zevach Pesach, p. 112)
The Egyptians had the Israelites heat the bath houses and keep them clean. The Egyptians were struck with boils and could not bathe. (Shemot Rabbah 11:5)
Since the Israelites did not come home from work until late into the night, they could not be with their wives. The Egyptians were struck with boils and therefore could not engage in intimacy either. (Abarbanel on Exodus 7:14)
The Israelites had to plant all of the trees and crops and could not go home. Therefore, the plants were destroyed by the hail. (Shemot Rabbah 12:3)
The Egyptians beat the Israelites, so the hail beat the Egyptians. (Abarbanel on Exodus 7:14)
The Israelites were forced to plant the grain in Egypt. Therefore, the locusts came and ate up the grain. (Tanna D'vei Eliyahu Rabbah, Ch. 7)
The Egyptians stole the Israelite's crops, so the locusts ate the Egyptians' crops. (Abarbanel on Exodus 7:14)
Death of the Firstborns
The Israelite baby boys were drowned in the Nile, so the Egyptian firstborns were killed. (Tanna D'vei Eliyahu Rabbah, Ch. 7)
Israelite children were mixed in with the mortar and used in the bricks to build walls. Therefore, the Egyptian firstborns were killed. (Pirkei D'Rabbi Eliezer, Ch. 48)
I encourage everyone to find more answers and either add them here or add a new answer yourself.
In addition to the considerations given by ezra realted to the treatment of the Hebrews by the Egyptians, each Plague was associated with one or more Egyptian deities. The Egyptians worshipped that deity, so the emblem of that deity caused them harm.
From that page, here are the deities being judged (ridiculed for their impotence and inability to save the Egyptians) by each plague:
1 - Water to Blood • Khnum - Guardian of river’s source. • Hapi - Spirit of the Nile. • Osiris - Nile was his bloodstream.
2 - Frogs • Hapi Frog goddess to Egypt. • Heqt Both related to fertility
3 - Lice • Seb The earth god of Egypt
4 - Flies • Uatchit - The fly god of Egypt.
5 - Cattle • Ptah • Mnevis Egyptian gods associated • Hathor • Amon bulls and cows.
6 - Boils • Sekhmet - Egyptian goddess of Epidemics • Serapis • Imhotep - Egyptian gods of healing.
7 - Hail • Nut - Egyptian sky goddess. • Isis & Seth - Egyptian agriculture deities. • Shu - Egyptian god of the atmosphere.
8 - Locusts • Serapia - Egyptian deity protector from Locusts
9 - Darkness • Re • Amon-re • Aten • Atum • Horus - Egyptian sun gods. • Thoth - Egyptian moon god.
10 - Firstborn • Pharaoh himself (revered as a deity)
God has consistently shown His true "Authority of Godhood" by opposing man-made idols on their turf. The Egyptians worshiped many gods, chief among them Pharaoh. God specifically targeted the top ten (he has a thing for numbers, 10 commandments, ten fingers and toes, etc.) to show His direct Authority and Power over and above each god. He took out the Nile gods, which gave life to Egypt. He had authority over the Frog god Heket, over the magic of Isis, the gods of health, beasts, grain, and vanity etc. He overtly displayed commanding absurd authority over every major tenant in Egyptian philosophy and cultural symbol, even daylight(Ra), culminating in the Authority over death of even Pharaoh's child.