A channel trailer is a tricky thing. It is important to put your 'best foot forward,' that is to say that if someone comes to your page to see your work you have mere seconds to really grab their attention and convince them to subscribe. That is why it is important to have as many videos as possible (a full channel reflects someone who works hard and may be worth subscribing to) a colorful and informative channel banner (because lets face it, how many channels have YOU subscribed to that don't have a channel banner) and a channel trailer. This is one short opportunity to show your work to someone before they click on a video, and you want to sell them on a subscription IMMEDIATELY. Asking them to subscribe is important, but more important you want to show them the absolute best in what you do.
If you really nail an awesome trailer that's great... but another approach is to look through your own catalogue. Find the video that gets the most subscribers and make that your trailer. it's already working better than every video you've made, right? So you want to use that as your hook to get someone to subscribe. If your channel trailer can beat that then use a trailer, but if not use the video with the most subscribers.