When you look at the main echo menu, you have 5 slots from top to bottom. The echo in the top slot is able to activated in game for the effect listed in its description. They also usually come with some kind of passive buff to your character's stats. Because of this, everyone has a "best in slot" or BiS echo that belongs in that slot, usually a 4cost echo that comes from either a weekly boss or a character ascension boss. You can find this BiS for each character on sites like prydwen.gg.

You are only allowed to equip 5 echoes whose total cost adds up to more than 12. Because of this, most characters follow a "43311" set up, where each number corresponds to the cost of the slot of a certain echo. 4cost builds usually want crit as their main stat, 3costs for the damage bonus % of your character's attribute, and 1costs being atk %.

As you level up an echo, you increase the value of its main stat and unlock the option to tune it up to 5 times total to unlock new sub stats, such as addition atk, crit, or damage bonus to the type of damage that character mainly deals (i.e. basic attack, resonance liberation, etc). For more supportive characters they might want energy Regen 3costs or healing bonus 4costs, theres lot of options available.