a) the simple past emphasizes the completed action;
b) the past perfect gives the sense that you're telling a story about a completed action, but that action is still relevant, either emotionally or because you are going to continue talking about it.
If you want to give the action even more current relevance (say, if you're still really excited about it), you could use the present perfect:c) Today was the first time I have seen a 22 Degree Halo, Rainbow around the sun.One other comment, not related to your question: Are you sure you are using capitalization and the comma correctly in these sentences? I'm not familiar with whatever scientific terminology is used here, but if I were to write the sentence myself, I would write it this way:
Today was the first time I've (ever) seen a 22-degree halo rainbow around the sun!
Interested in Language