To me a great sign of friendship is someone going out of their comfort zone to make you happy. For example, a hug from a friend who is typically aloof with people, or a friend who hates the outdoors going on a hike with you. They're subtle kind acts that from some people may not mean a lot, but when they come from a certain friend who you don't expect it from it's very heartwarming.
If you want to be really dramatic then taking a bullet (in the literal sense) for your friend is pretty nice, but most of us won't find ourselves in that situation thankfully.
should i answer this? someone who quotes themselves in their own signature is likely to be a bit of a jerk, so i'm not sure i want to give you the opportunity to rip my answers to shreds in the replies
Here's my blunt statement: In some cases, best friends usually joke, insult, and roast with each other. In the end, especially during a dire situation, they will be on your side no matter what.