I use to be as big fan of GT, never really understanding why so many fans hated it, but in recent years or so I've come to realize what those fans were talking about and now agree with them on how bad GT was and, like many fans, I no longer count it canon.
For Digimon, personally I think everything up to Digimon: Frontier (Digimon: Adventures 1 & 2, and Digimon: Tamers) were the best series of the show. After Tamers I feel that was when the show began to go downhill.
So, out of these choices, I'm going with Digimon: Tamers as the best and GT the worst seeing, as far as I can tell, most fans liked Digimon Tamers while a majority still don't count GT as a canon.
Asked the same question on a couple of other forums and many would agree that Digimon Tamers beats DBGT, and that DragonBall GT flops and fails while most everyone (including myself now) agrees Digimon Tamers knocks DBGT and the succeeding Digimon series' out of the Park