Merry or Pippin
 

I can never decide who is the best of these two hobbits. Merry or Pippin.

I suppose that depends on what you mean by "best"...

Do you mean most effective in helping in the Ring quest or some personal qualities, or something else entirely?

Such a broad question begs for a broad answer: Merry.

Definitely Merry.

The fool of a Tuk of course :smokin:

I have always considered Merry to be the better, most mature and thoughtful of the two

In the early chapters, Merry is the planner. It's Merry who makes the house at Crickhollow available, it's Merry who arranges for Fatty Bolger to stay in the house to give the impression that Frodo stays there and it's Merry who organises transport for the party and provisions for the journey. He gives thought to the little things too, like having three baths ready so that Frodo, Sam and Pippin don't have to wait their turn to get cleaned up.
As shown by his comments after he and Pippin escape from Saruman's orcs, Merry spent time in Rivendell studying maps while Pippin just enjoyed himself.
After Merry and Pippin are separated, Pippin offers his sword to Denethor on impulse; Merry gives his fealty to Theoden only after some thought, because he has come to love the old King.
In the closing chapters, Pippin plays the impetuous Lieutenent to Merry's thoughtful General. Pippin rushes off to gather troops from Tookborough while Merry takes command at Bywater, planning his campaign with Farmer Cotton.

We should not be surprised that Merry shows the most maturity of the four Hobbits of the Fellowship. Sam, before the War, had little responsibility other than looking after the garden at Bag End. Frodo, with his inherited wealth, had nothing to do except study Elvish, write poetry and dream. Pippin was too young (under 33) to take on the burden of being heir to the Thain. Merry, however, was of age and would have been expected to take part in the running of Buckland. He was being trained in organisational and leadership skills in a way the others were not.
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Merry definitely did more to influence the War of the Ring...
In the movie you might be decieved since you see Pippin light the beacons, but in the books it's clearly Merry that does more...
Pippin seems to see it like that as well because as he is sitting in front of the Black Gate with the Host of the West he says to himself:

Pippin did indeed play an important part in saving Faramir, but this was not so important for the outcome of the War of the Ring, since Faramir would no longer play an active part in the events. It was indeed his foolishness that brought a great deal of luck to the Fellowship since he could see a part of Sauron's plan in the palantir.

Still, Merry was clearly more important as he was the one who made it possible for Eowyn to kill the WK. Had this not happened, the events that took place afterwards might have been very different.

I was always more of a Pippin fan, but recently I've liked Merry just as much. If you are going to talk character development wise, you would have to say Pippin is the greater. He is transformed from a know-little Hobbit to a soldier of Gondor, coming back to the Shire no longer the youth he was. He went into the journey a "boy" if you would, making many mistakes and follies along the way, but returned as a "man".

Merry was thoughtful and clever all along. He usually knew what to do, and what he wanted to do. He rarely (if ever) made a mistake. So if you're talking solid character wise and overall contributor to the Quest, then perhaps you'd say Merry. There's plenty Merry did, but Pippin also helped the Quest greatly even though he was the one who didn't mean to. In looking through the Palantir, Pippin gave the Company the desperate answers they needed. Not to mention he saved Faramir. Can you compare that to aiding in slaying the Witch-King? I don't know, in the overall mission of the Quest, probably not.

I like Pippin the most. Though Gandalf is my favourite charachter

I can't really compare them. They're different, they have different gifts, and they were both important to the quest.

Although I kinda like Pippin better.