Giving my 2 cents on Junior
Clown Cannon - This is situational. It doesn't travel very far if you don't charge it and it isn't very fast.
I only use it trying to edgeguard, rarely got some kills with it but don't count on it that much.
It's not very useful but not that great either. I prefer the smaller one 'cause it's faster.
Piercing Cannon - This is probably the best of the 3. It does less damage and the hitbox is not that big, but it goes far even if you don't charge it. The only really bad downside is that it cannot be spammed consistently.
If you don't know which one to use, this is the best bet.
Air Cannon - this is just terrible. You have to charge it almost to the maximum to have a decent effect and the hitbox is not even that big. Could be useful against Lil'Mac or a char who doesn't sweetspot the ledge. There's really no point in taking this.
Clown Kart Dash - The best for damage and mixup. If it hits it puts your opponent above you and you can follow up with nair, fair, dair or uair based on the opponent's %. It helps a lot with Jr's recovery and has the highest jump of the 3.
Koopa Drift - Travels farther than the first when you drift and has multihit hitbox, so it's good against roll spammers. The downside is that it doesn't jump as high and when hits during the run puts the opponent in front of you, so you can follow up only with fair or nair. Could be useful for edgeguarding characters that recovers more orizzontally but the first one is annoying as well as this, has more damage output and allows for more mixup. This move is kind of like Sonic sideB with lot less damage output.
Grounding Dash - Doesn't travel far and has startup before the run where it goes backward a little.
It grounds opponents when they're on the ground, allowing a kill at high %. If your opponent sees it starting he can easily shild or jump away. I personally think it could be useful against matchup where landing a kill is a tad difficult.
All 3 sideB eat your second jump if you jump cancel during a run, so double jump->SideB when recovering for the best effect.
Abandon Ship - Probably the best. Could combo into the hammer at certain % securing a kill, could be used offstage from above without any risk for damage or stage spiking and is the best of the 3 for recovering.
Meteor Ejection - Doesn't go as high as the first one but can meteor the opponent if he has the koopa clown on his face when it starts.... except that the explosion hitbox could cancel the meteor effect and really, don't count on it that much for a meteor, it has super armor at the start but you must go offstage on the face of your opponent for it to work. It's not that great on the ground either, I personally think it's easier to combo into the hammer with the first one, and the first one even kill by itself.
Koopa Meteor - Meh, I personally think this is just terrible. It doesn't sweetspot the ledge when it goes upward, so you can get punished really hard just for using it. It's not even that great to hit with because the hitbox only starts when going backward.
Mechakoopa - This is definetely the best for controlling the stage. You drop one, see if the opponent gets hit by it, shield, jump, grabs it and you play accordingly. You can even put some strings if you hit with aerials. The downside is that reflectors turn Mechakoopas against Jr and it's not that great against those chars.
Impatient Mechakoopa - This is a simple spammable projectile. It's great when you throw one and the opponent grab it in the air because he imputted an aerial or an air dodge, it will explode right in their hands. I don't find it useful as the first one because it doesn't give stage control, you can spam it from afar or try to prevent the opponent from getting back to the stage.
Big Mechakoopa - This version doesn't attach to opponents, it's slower and has a short fuse. I find it just situational: it's useful against people with a reflector because the fuse is so short that it will not get back to Jr and could potentially hit the opponent even if reflected because of the huge hitbox, and against chars with spamming projectiles because the mech is so big that will cover Jr entirely.
It is even easier to do the double damage bug because of the short fuse.
I usually go for 2-1-1-1 and sometimes I change the Mechakoopa according to the matchup I'm playing.