I will get to this, but if you want a quick and dirty guide to what you should go watch, here it is:
1. The Switchman (the pilot)
2. Seige
3. Cypher
4. Flight
5. Warriors
These are my five favorites.
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Then, just because they are the episodes that launched a thousand fics or more, you need to watch:
1. Sentinel, Too, parts 1 and 2
2. The Sentinel by Blair Sandburg
And try to get your hands on the Sentinel blooper reel so you can see where all those kisses in the songvids come from. LOL.
Although, be forewarned, these episodes aren't easy to watch.