In general, Tier 1 abilities are stronger than Tier 2 or 3 abilities. They may be used once per encounter. To create a Tier 1 Ability, imagine something that is your character’s “signature move”. What are they capable of doing that is awe-inspiring? It could be a devastating attack, an incredible healing ability, or even a jaw-dropping utility-like ability. This shouldn’t be something that makes your character godly, but it should be a defining feature of who they are.
Tier 1 abilities for Spellcasters are specifically focused around utility and flavor. There are no damage / healing charts for creating Tier 1 abilities. You should use your imagination to come up with an awesome, character-defining ability.
If you're considering a Tier 1 ability and trying to imagine how much damage it should do, a loose guideline is dealing maximum damage with your basic attack to all nearby enemies (or an AOE defined space); or deal maximum damage with your basic attack +5 to a single target.
All Tier 1 abilities must be made with DM approval to ensure that they are somewhat balanced.
Some characters forgoe having a Tier 1 ability to create a special, rule-bending ability to be used out of combat. See the Ability Compendium for examples. The Level 10 Perks page has extra information about customizing high level characters.