Anything you can chose between giants and archers as they are the most useful troops in clash of clans and are pretty cheap too. So you should upgrade them first.
1500000
2,750,000 elixir, but you get witches.
Upgrading a cannon to level 7 in Clash of Clans takes 5 days. Additionally, you must have Town Hall level 7 to unlock this upgrade. During this time, players can continue to gather resources and work on other upgrades.
2,000,000 elixir (2 Million)
In clash of clans, the maximum level on balloons is level 8
At level 1 it costs 15000. It increases by 3500 every upgrade.
To upgrade a level 4 Archer to level 5 in Clash of Clans, it costs 25,000 Elixir. Players need to ensure they have the required resources and that their Barracks is at the appropriate level to complete the upgrade.
To upgrade the Barbarian King to level 5 in Clash of Clans, it costs 40,000 Dark Elixir. However, if you choose to use gems for the upgrade instead, it costs 1,200 gems. Keep in mind that upgrading the Barbarian King also requires a level 10 Town Hall.
Level 15 walls do not exist in Clash of Clans, although they might in a future update.
The price goes up some per level. The first upgrade is 10,000 dark elixir. By the time you reach level 40, it will be 190,000.
It takes six days to upgrade to Town Hall level 7.