What is the current objecvtive ranking?

I don't know what the general consensus is but if you can get dragon or a tower, I'd say get dragon. The tower is always up, dragon isn't. Then I'd say go for the Nexus Turrets over Baron if the enemy team is down enough players/significant respawn timers as why waste time getting an objective if you could just win?

Nexus > Inhibitors(2+)> Baron/Dragon 5 > Nexus Towers > Inhibitors(1) > Inhibitor Towers > Inner Towers > Dragon > Outer Towers

General guide, though the game course could change; for instance, if they're going for baron but you have no tank or minions near towers going for a dragon (even if it isn't a 5th one) would usually be more useful.

This also assumes these are the ONLY objectives you can get; obviously, getting both Nexus Towers and being able to get their Nexus would make it the best, or taking 2 towers could be worth more than contesting dragon (unless it's 5th dragon, then get 3+ towers or a couple inhibs).

1. Nexus
2. Clear all 3 inhibitors
3. Get 5th Dragon when tied 4 dragons each (don't think it would ever be this close, but taking your 5th while denying their fifth is huge).
4. Clear 2 inhibitors
5. 5th Dragon when opponents have 0, or deny 5th Dragon by taking any Dragon
6. Baron
7. 5th Dragon when opponents have 1 but fewer than 4
8. Nexus towers
9. Inhibitor
10. 4th Dragon (creates pressure of 5th Dragon)
11. Inhibitor turret
12. 1st Dragon (ap and ad bonus is huge, also reduces threat of 5th Dragon greatly)
13. Inner mid tower
14. Inner long Lane turrets
15. 2nd/3rd dragons
16. Outer mid tower
17. Outer long Lane towers.

The first 3 should be obvious (win, then 3 inhibitors at once I've never seen lose, then the dragon explained above). 2 inhibitors is worth more than Baron/5th Dragon as if you turtle well you should be able to give up only 1 inhibitor generally, or if you're trading objectives the 2 inhibitors will really alleviate pressure in those lanes so you can group 5 in the other Lane.

8 and 9 should be obvious. I like the pressure that 4th Dragon creates, but only if you're sure you'll last until the next Dragon spawns. The threat of 5 dragons only appears after it spawns. Inhibitor turrets are obviously next. Your first drag is better than inner turrets as if you're taking inner turrets and don't have a dragon yet you're probably down a lot in Dragon count and they could be going for 5, not to mention the boost for 1 is the best after 5.

Mid Lane towers (outer and inner) are more valuable than the respective long Lane towers because they offer faster access to taking an inhibitor if you win a teamfight later. Otherwise the rest should be self explanatory.

All of this is just my view and completely theoretical. Obviously I don't have enough Games played to give statistics on how these win Games, and competitive play could show me to be completely wrong :p