The purpose of a reel is to show you have talent in your classification. Search the internet for other people's reels in your classification to see what they are doing (preferably working people. you can usually find their reels on their agent's websites, or their own) and model the ones you like.
Another tip is to think long term. If in 5 years you want to be directing romantic comedies and you have 3 shorts that are all different genres, you may want to consider doing shorts that are romantic comedies, because:
1. The more you show in your genre, the more it proves you're an expert
2. You get better with each project so you want your best work to be in the genre you're targeting
3. You'll be creating relationships with people who enjoy the same type of work you do
The objection I get to this is, "But I can direct anything and I want people to see that so they'll hire me." My answer to that, is that early on you will be hired based on the relationships you create, so it really doesn't matter what genre is on your reel, what matters is how well you can sell yourself. LONG TERM, it will matter what genre is on your reel, so target the types of films you want to make.
For more tips and articles by top entertainment industry career coach, The Greenlight Coach, visit www.theGreenlightCoachBlog.com