Isaac's physics
Dash speed: 7/10 (Ganondorf = 1/10, Sonic = 10/10)
Weight: 4/10 (Jigglypuff = 1/10, King Dedede = 10/10)
Falling speed: 6/10 (Jigglypuff = 1/10, Captain Falcon = 10/10)
Air speed: 3/10 (Luigi = 1/10, Yoshi = 10/10)
Normal moves
Jab: one punch, one knee, one sword slash, and one Gaia psynergy attack. The Gaia attack is obviously the most powerful attack from this jab, and has a decent horizontal knckback.
Forward tilt: a Thorn psynergy attack. A thorn quickly emerges from the ground just forward Isaac, then disapprears. Decent damage and diagonal knockback.
Up tilt: a Spire psynergy attack. The spire appears above just Isaac himself, and if there is an enemy in the area where the spire is supposed to appear, then it immediatly brokes and inflicts significant damage and knockback, otherwise the spire falls on the ground and becomes weaker.
Down tilt: a Quake psynergy attack. Inflicts low damage, but has a decent horizontal knockback. Enemies can only be hit if they are on the ground.
Dash attack: a Ragnarok psynergy attack. Isaac jumps, summons a giant sword, and sticks it in the ground, causing a localized explosion. This move has a massive knockback and damage power, but a LOT of ending lag and is easily punishable.
Neutral aerial: Isaac rotates on himself using his sword to slash enemies. Semi-decent vertical knockback.
Forward aerial: a vertical sword slash. Decent damage and horizontal knockback, but if the enemy is slightly under Isaax, then the knockback will become horizontal instead, and send the enemy towards the ground.
Up aerial: Isaac points his sword skyward. Inflict good damage, below-average vertical knockback, and has sex kick properties.
Back aerial: a Rockfall psynergy attack. Isaac looks back to summon a few rocks behind him, but they quickly disappear (no they don't reach the ground). It's a multi-strike move, and the last rock summoned obviously has a higher knockback, being mostly vertical.
Down aerial: Isaac rotates on himself using a sweeping kick. Low damage, but above average knockback, and Isaac's move with the most horizontal knockback.
Grab attacks
Grab: a Catch psynergy. Isaac summons a giant hand forward him, and can catch an enemy. This move has a very high grab range, similar to Toon Link; actually it's probably the non-tether grab with the higher range.
Pummel: the enemy in crushed in the giant hand summoned by the Catch psynergy. Each pummel inflicts high damage, but has also a high ending lag, meaning that you cannot pummel your enemy so many times before being forced to release it.
Forward throw: a Move psynergy. The animation is exactly the same as Isaac's Assist Trophy appearance in Brawl, but it's faster, the distance covered by the hand is lower as well, and now it actually inflicts damage and vertical knockback once the hand disappears.
Up throw: a Lift psynergy. A second hand appears and the enemy is lift, then when they disappear it gets damage as well as vertical knockback.
Back throw: the catching hand goes behind Isaac, disappears, and Isaac quickly kicks the enemy like how Link kicks Pit during the Palutena trailer. Higher damage but lower knockback than his forward and up throws.
Down throw: the catching hand crushes the enemy on the ground. This throw inflict powerful damage, but has a poor knockback.
Special moves
Isaac has an unique gimmick that allows him to gain summon points each time he uses a special move. However, if he uses a given special move, he cannot gain another summon point from this same move until he spend it on a Smash attack, because Isaac's Smash attacks are summon-based.
For some of those djinn I do researches on the Golden Sun wiki, looking for TLA and DD ones as well. I have trouble to imagine a moveset using only the djinn I found myself on the first game lol.
Default neutral special: Flint. It's a storable charging move, much like Donkey Kong's Giant Punch. Once he attacks, Isaac attacks with a powerful slash from his sword, with an emanating orange aura. Decent knockback, but this move is more notable for its very high damage power, inflicting 33 % when fully charged. This move also gives one Earth summon point.
Custom neutral special 1: Sleet. This move is similar to Marth's shield breaker in both damage and knockback, but it charges slower and is storable. This move also gives one Air summon point.
Custom neutral special 2: Scorch. Similar to Flint, it's a charging move, but with even more damage power and a low fixed knockback. If used when fully charged, it inflicts 40 % damage and buries enemies. This moves also gives one Fire summon point.
And for the rest of the special moves... okay I'll stop here for now. As I already said I have only played the first Golden Sun game and I don't have all the djinn. If someone with more GS experience than me could suggest some djinni-based special moves that would be cool!
Smash attacks
This is the most interesting part of Isaac's moveset. When he uses a Smash attack, he will spend summon points gained using special moves. Each of his special moves has a three different variations, each of them being based on different elements. Due to this, some summons cannot be done with a given special custom moveset. If different summons can be done with a given Smash attack and with the same summon points gained, then there is a priority list. If he can't summon anything using a Smash attack, he will intead use a sword-based attack, a rather weak Smash attack unless completely charged.
Note that I spoiled myself about some of TLA summons on the Golden Sun Wiki due to multi-element summons not existing in the original Golden Sun.
Forward Smash
High priority:
Moloch. Requires two water and one air summon points. The Moloch beast appears behind Isaac, but it's actually a ghost with no hitbox. When using this Smash attack, Isaac sends an icy wind toward himself. It's a multi-strike move that freezes any enemy caught, and the final blows inflicts very strong horizontal knockback.
Medium prority:
Venus. Requires one earth summon point. A multi-strike move. Isaac attacks forward with an animation similar to the Venus summon animation in Golden Sun. The finishing strike has a slightly above average knockback for a Smash attack, and this knockback is vertical.
Low priority:
Mercury. Requires one water summon point. A multi-strike move. Isaac attacks forward with an animation similar to the Mercury summon animation in Golden Sun. The finishing strike has a slightly above average knockback for a Smash attack, and this knockback is horizontal.
No possible summon: Isaac attack with a vertical sword slash forward him. This Smash attack inflicts horizontal knockback, but is rather weak if used uncharged.
Up Smash
High priority:
Flora. Requires two air and one earth summon points. Flora appears in the background, and a vortex of petals appears above Isaac. The vortex itself is a multi-strike move that tends to trap enemies and to send trapped enemies to the center of the vortex. It quickly disappears, and the more a trapped enemy was in the center of the vortex, the more horizontal knockback it takes.
Medium priority:
Megaera. Requires one fire and one air summon points. Megaera appears in the background, and four swords appear from the ground to form a square, but those swords doesn't have any hitbox and what hurts ehe enemies is the explosion that quickly appears in the square formed by the swords. It's a one-strike move with heavy diagonal knockback.
Low priority:
Jupiter. Requires one air summon point. A multi-strike move. Isaac attacks upward with an animation similar to the Jupiter summon animation in Golden Sun. The finishing move has a slightly above average knockback for a Smash attack, and this knockback is vertical.
No possible summon: Isaac rotates on himself and attacks pointing his sword skyward. This Smash attack inflicts vertical knockback, but is rather weak if used uncharged.
Down Smash
High priority:
Cybele. Requires three earth summon points. Cybele appears on the background, then vine grow from the ground. This move inflicts massive vertical knockback if the enemies are in the ground when it is used.
Medium priority:
Zagan. Requires one earth and one fire summon points. Zagan appears behind Isaac and sweeps the ground with a giant axe. Very strong horizontal knockback.
Low priority:
Mars. Requires one fire summon point. A fireball appears above Isaac, can inflict low damage anyone who enters in contact with it, and quickly falls into the ground to inflict fire damage to anyone on the ground and close to Isaac. The impact inflicts slightly above average knockback for a Smash attack, and this knockback in diagonal.