For me shaco´s biggest strength is his flexibility. He is a great 1st pick because you can adapt to every matchup by going ap or ad. You can also "play your own game" by ignoring the enemy jungler, taking his stuff top-side when he ganks bot, taking herald when he takes dragon etc. etc.

Ap and ad shaco have two completely different playstyles.
Being able to play both ad and ap, knowing when to go which can increase your impact on the game significantly.

I always ask myself after the game how the game would have been if I would have been playing ad/ap.


AD SHACO:

With Ad-Shaco you are trying to snowball, assassinate carries, always looking for flanking angles and surprise the enemy adc/apc. You have a hard time escaping when your q is on cooldown and you are quite squishy, so choose your engage wisely.

Choose Ad shaco if the enemies have an ranged champion as their win-con (typically an mage or adc) that you can kill. Someone else has to deal with the frontline, so you want that teammate fed if possible.

AD Shaco is strong early and mid and falls of late. You can compensate that a bit with going IE and LDR late. Keep in mind that ad-shaco is pretty item-reliant, so it´s gonns get though if you fall behind.


AP SHACO:

Ap-shaco is kind of an "zone control" "disruptive" "disengage" champion. Pretty strong against melees, especially engage champions. Box placement is his bread and butter, because the fear + dmg can decide teamfights by its own. You constantly have to think about where the enemy could be in 30sec, preparing the area with boxes (drake and baron fights are quite obvious).

Choose ap shaco if your team lacks ap damage, the enemies have strong engage and melee champions.

Ap shaco scales pretty good but has a hard time early. You can compensate that a bit with mejais, if you are confident in not dying.

You can win a lot of games by tilting the enemy. Shaco is the most hated champ in league so have fun with it. A good way is to emote after every kill or enemy mistake, I can recommend the "penguin hat" emote or the classic "thumbs up". A little dance or laugh can help to.

Keep in mmind that shaco can splitpush and backdoor quite well.

Also drake and baron steals are possible.