The GT300 series is toughest, if you use the cars the AI uses: Cusco Impreza, Amemiya RX-7, BP Trueno, Momo MR2, Daishin Silvia, Xanavi Silvia, Weds Celica, Trampio FTO...should yield a good championship battle. With the exception of the Celica and the MR2, it's a tight battle.
I'm sure most everyone used a car with too much power the first time out (me included); but an R30 Silhouette or S2000 GT1 isn't much of a challenge....anything from 460hp on down makes for a good race, though!
I think the GT500 championship is tougher than the World League, unless the GT-One shows up. Then the championship more or less goes down to the wire. The GT500 cars stay in a long formation; never separated by more than 3 seconds at the flag. So a last-lap spin will cost you; whereas a late mistake in the World League means that Lister Storm or Alfa 155 TC may still have a tough time catching you!