Favourite a toot if you like it, that lets the author know you liked it.
Boost it if you want to share it with your followers.
Reply if you have something to say to the author or on the topic. (You can set your reply's visibility to mentions only if you want to reply to just the author.)
Boosts have the added advantage that they spread the post to all the servers of all your followers, so the post will become searchable on all your followers' servers too (if it wasn't already).