What is the biblical argument against the claim that Jesus is just "a god", but not "God Almighty"? As I understand it, Jehovah's Witnesses teach that Jesus is a god, but not God Almighty.  However, this goes against the teaching of the majority of Christian denominations and certainly evangelical denominations.

What, then, is the biblical basis for the claim the Jesus is not merely a god, but is, in fact, God Almighty?
I love Narnian's answer indicating that Jesus is the Alpha and Omega. Here are a few other proofs to supplement that answer.

1. There is only one God


  "I am He, And there is no god besides Me" -Deuteronomy 32:39
  
  “You are My witnesses,” declares the Lord,
  “And My servant whom I have chosen,
  So that you may know and believe Me
  And understand that I am He.
  Before Me there was no God formed,
  And there will be none after Me." -Isaiah 43:10
  
  Thus says the Lord, the King of Israel and his Redeemer, the Lord of hosts: "I am the first and I am the last, And there is no God besides Me." -Isaiah 44:6
  
  “I am the Lord, and there is no other; Besides Me there is no God." -Isaiah 45:5
  
  "there is no other God besides Me, A righteous God and a Savior; There is none except Me." -Isaiah 45:21


God went to great lengths to clarify that there was only one God in all of existence. I see two possibilities:


There is only one God in all of existence, or
God just wants everyone to believe that there is only one God in all of existence! (I say this tongue-in-cheek, of course; God is not a deceiver; there is indeed only one God.)


Either way it seems extremely foolish to teach anything other than "there is only one God in all of existence."

2. God Almighty: Lord: Savior: Jesus

The verses just cited also indicate that the one and only God is the Lord and Savior, but here is further support:


  when Abram was ninety-nine years old, the Lord appeared to Abram and said to him,“I am God Almighty" -Genesis 17:1
  
  “I, even I, am the Lord, And there is no savior besides Me." -Isaiah 43:11
  
  I have been the Lord your God Since the land of Egypt; And you were not to know any god except Me, For there is no savior besides Me. -Hosea 13:4


Here we see that God Almighty is the Lord, who is the only Savior. (Note: A number of passages, including Nehemiah 9 prove that the same "God" is being referenced in both of these passages.)


  For the grace of God has appeared, bringing salvation to all men, instructing us to deny ungodliness and worldly desires and to live sensibly, righteously and godly in the present age, looking for the blessed hope and the appearing of the glory of our great God and Savior, Christ Jesus -Titus 2:11-13
  
  grow in the grace and knowledge of our Lord and Savior Jesus Christ. To Him be the glory, both now and to the day of eternity. Amen. -2 Pet. 3:18


Our great Lord, God, and Savior is Jesus. If Jesus is not God Almighty, these passages would be blasphemous.

3. The blood of God


  Be on guard for yourselves and for all the flock, among which the Holy Spirit has made you overseers, to shepherd the church of God which He purchased with His own blood. -Acts 20:28


Given that there is only one God, and given that He purchased the church with His own blood, and given that it was Jesus' blood, we have yet another proof that Jesus is that one true God.