For a long time I tought that Gragas was a week and useless champ, but upon buying him I understood that he can be played in many different ways which makes him a good adition to any team.
The reason I am making this build is because I've seen many Gragas builds and I think that if I combine the ideas of the other builds I can come up with something fantastic and here it is. I've made this build so you can play gragas anywhere on the map, either top, mid, bot(except jungle).

Gragas has quickly become a favourite champ of mine because you can choose to build him into anything: Tank, AP carry, AD offtank, AP offtank.
Before asking 'should I stick to the build in any case or I can buy other items' you should know that this build is made for my personal gamestyle, you can buy Lich Bane instead of Will Of the Ancients or Zhonias hourglass.
Boots! Boots can be really hard to decide when it comes to Gragas. Although I prefer buying mercury's threads or Sorcerer's shoes doesn't mean you can't buy Ionian boots or ninja tabi or even boots of swiftness. It is all up to your decision.

Playing Gragas can be fun and easy to master. Here is the combo you should always use if you are still new to Gragas: W(before you attack you should always use this abillitie)->E->Q->R

After you've mastered this combo you can start experimenting. For example: R(to knock them into you)->E(to slow them so they can't escape)->Q(to finish them off)

You're Q is very powerfull if you can land it right, that's why you have E so you can slow them and land your Q easly.
Many people underestimate Gragas so they will often try to take you down as quickly as possible, so if they attack don't be afraid to use your combo(remember! your E also stacks AD, so using W before using E can help a lot).

Remember that your passive restores 2% of your max health upon using an abillitie so use your W anytime you can to maximise your sustain and minimize the dmg you will take.

I can't point out how important your ulti is. With it you can desrupt your enemies. Throwing your ulti in the middle of the fight won't help much so try and knock out the most dangerous or squishy target so you can take it out. That will give you a huge advantage over your enemies because by throwing your ulti not only do you deal a lot of damage but you also out number them by taking out that one target you knocked back from his/her team.