You can use dispel magic to end one ongoing spell that has been cast on a creature or object, to temporarily suppress the magical abilities of a magic item, or to counter another spellcaster's spell. A dispelled spell ends as if its duration had expired.
If you want to dispel a spell from a creature, you must not target the creature itself, attempting instead to end one discrete magical effect (spell) affecting one creature. The spell you're targeting only ends on one creature or object it is affecting.
If the effect of a spell becomes permanent, it can be dispelled, unless its description says otherwise.
Wish CANNOT be affected by Dispel Magic; there's no ongoing magic to dispel.
A spell that has an instantaneous duration simply cannot be dispelled regardless of what level dispel magic is being cast at. Furthermore, the definition of Instantaneous spells states (emphasis mine): Many spells are instantaneous.
You can use dispel magic to end ongoing spells that have been cast on a creature or object, to temporarily suppress the magical abilities of a magic item, to end ongoing spells (or at least their effects) within an area, or to counter another spellcaster's spell. A dispelled spell ends as if its duration had expired.
The simplest reading of the way dispel magic works is that the only time casting dispel on the caster is useful, is when the caster is the target of the spell. In other words category 1-5 cannot be dispelled by casting dispel magic on the caster (if the caster isn't the target).
701.5a To counter a spell or ability means to cancel it, removing it from the stack. It doesn't resolve and none of its effects occur. A countered spell is put into its owner's graveyard. 701.5b The player who cast a countered spell or activated a countered ability doesn't get a “refund” of any costs that were paid.
It's perfectly within the rules to wish the tarrasque to die, or to be in the center of the sun, or into another planeof existence, and it would work fine.
You can't can't dispel magic on “any and all invisibility effects in range” because that's not how targeting works — that's not one, as dispel magic requires, that's multiple.
As the Hallow spell is of Instantaneous duration, it may not be dispelled by Dispel Magic, which require a finite duration (even if sad duration is Permanent).
Can You Rebound a Spell that Exiles Itself? No, you can't rebound a spell that exiles itself. Rebound doesn't get a chance to apply if a spell moves itself into another zone as part of its resolution (e.g., Arc Blade, All Suns' Dawn, Beacon of Unrest).
You cannot dispel an endless spell and cast it again in the same hero phase. The same applies to invocations banished in the same turn. Endless spells are moved at the end of the hero phase, in each hero phase (rather than at the beginning of the battle round).
You can't normally dismiss a spell that you cast unless (a) its description says you can or (b) it requires concentration and you decide to end your concentration on it. Otherwise, a spell's magic is unleashed on the environment, and if you want to end it, you need to cast dispel magic on it.
You undo a single recent event by forcing a reroll of any roll made within the last round (including your last turn).
There is no in-game way of removing learned spells, but if you're playing on PC you can use the Spell Eraser mod. Adds a power that removes spells from your character. Some spells become obsolete as part of your character's progression. It's by design.
And in answer to the general question, Tiamat, especially with the new statblock from Fizban's. Tiamat has a 26 int and wis, the tarrasque has a 3 and 11. Tiamat has plans that go well beyond what a mortal could comprehend. She's much more dangerous than the tarrasque, which is just crush, kill, destroy.
The Tarrasque is an undead dinosaur-like God used by Akarot as a hollow body in his war on Bahumia.
One of these abilities is particularly worthy of note: Tarrasque can't be permanently killed unless you cast Wish or Miracle over his remains. Since you're not a level-17 caster, you're not going to be able to naturally cast either of these spells.
Counterspells are a great tool in Commander, and including a couple of the juicier ones at least in your decks is a good idea. It's possible to overdo it, though—if all you play is permission, you're not advancing your own board state; and there's no way you can keep up with all three opponents.
Yes, you can counter a hexproof creature.
A creature spell on the stack counts as a spell and not as a permanent. Hexproof is an ability that affects the card as long as it's a permanent, so you can absolutely counter it.
No, permanents aren't spells. While any card is on the stack it's a spell, but that spell can become a permanent once it enters the battlefield. Some cards, like tokens, only ever exist as permanents and are never spells.
The official rules expect a character at level 20 to have 6 permanent magic items. That's a perfectly manageable number, and you should be very cautious to go beyond that. The official rules only have one limitation on this problem: Attunement.
Bead of Force functions exactly like a Resilient Sphere cast at level 10. Both are subject to Dispel Magic.
Dispel magic does not remove the field, though Mage's Disjunction might. Two or more antimagic fields sharing any of the same space have no effect on each other. Certain spells, such as wall of force, prismatic sphere, and prismatic wall, remain unaffected by antimagic field (see the individual spell descriptions).