As an assassin if you're fed you can one shot enemies in lower elo. This is why I prefer going tank/bruiser with hydra as the only damage item, he is much more viable and useful in teamfights. You can zone an adc or APC away from a teamfight to let your team deal with the others. You need to play tons of games with him in order to understand how to pull off these tricks and make a mess out of teamfights. Check out the latest videos I added 2 days ago, in the media section - Teamfighting with shaco, and watch how I zone their carries and make an entire mess out of their team composition. That, and counter jungling, as well as securing objectives, is what makes him so fun and viable. Try my build you won't regret it, its in the 'personal build' section.

'Which'
They are all good. If you want to go tank bruiser I recommend trying out the Shaclone build. If you want to go Assassin, you have either AD or AP, both are good. Try them out and see what fits best to your playstyle.

Grasp of the Undying is only used on one of the builds. It's something I used back a few years ago, where you go crazy counter jungling. It's better for dueling and you get the extra mastery points from being tanky early game. Doesn't mean it's viable, but sometimes its really fun. Colossus is very situational since it only works if you trigger the fear from JitB. Grasp is a constant effect.

I posted a build with Titanic hydra as well just yesterday, its not bad but I think it lacks sustain a bit. It's good if you're really ahead. Anyways check it out, its called 5000+ HP - 300DMG - 45% CDR :P

Nice, be sure to check out the videos especially the Gold to plat montage to see tips and tricks that will help you regardless of the champion you pick :)