Worst I ever witnessed personally was a group of British (English actually) tourists on the train from Schiphol Airport to Amsterdam. They obviously were totally pissed, so louder and more obnoxious than usual. They smelled bad and were smoking. When the conductor made the rounds, it turned out they had no tickets and proceeded to verbally assault the conductor for "discouraging tourism" and other vague, intoxicated sentiments. And then how there is nothing in Amsterdam except drugs and prostitutes anyway... They were escorted off the train at Amsterdam Central.

If the conductor successfully discouraged tourism in this instance, more power to him. ^

I generally find the "best" tourists to be old American couples. I have never ever witnessed rude and loud or disrespectful behaviour from them. They generally may not agree with everything they see, but they have the tact to at least respect local sensibilities and culture, and to not make a fuss.