Well, towers are great, because gold wins games and it helps open their jungle up to countering and more unexpected ganks. These are best early: they snowball your team, as opposed to drag, which is minor until it gets major at x5.
Dragons are weak unless you or the other guy is going to get to 5, in which case they're vital: as a rule of thumb, they're meh when you're tied for dragon kills, good when one team is ahead (open the lead or close the gap), and critical when one team is at 4 (whether it's you or them).
Barons are both overrated and underrated. I've seen lots of team win teamfights in the 20-27 minute range that gave them an opportunity to baron, but they didn't take it. Don't assume it's a just late game objective. At the same time, baron just isn't that helpful in the classic 5v5 staredown at the losing team's mid inhibitor turret. The losing team's waveclear will generally keep minions back unless the winning team is strong enough to ignore tower: baron doesn't make much of a difference here. Baron wins games when the team that has it fans out to every lane to keep the buff on all minions. If you do that, and you can't win the fights you need to take towers, rotate a lot: eventually your team will be at a tower and the defenders will be too far to react, so rip it up, even if you can't stay long enough to take the inhib. When you're defending against this, the attacker's spread out, so try to pick/collapse on them one or two at a time. Conversely, when you're the attacker, your team has to be able to keep pressure on towers while staying safe. Eventually someone screws up, a lot of people die, and the game ends or the losers get closer to being even.