Most of the the arguments used against Tim Duncan are complete fallacies.
Some of them may hold water, but if they are used against other great players, will leave you in a position where you will not be able to find 10 guys ahead of him.
For example, the claim that Duncan played in an era of weak forwards. Patently false. The PF position has never been stronger in the NBA than in the recent decade with the likes of Duncan, Garnett, Webber, Rasheed, Dirk, Amare, Gasol, McDyess, Jermaine (healthy), Bosh, Brand, Boozer, Marion and several others I'm probably omitting. What's common to people in the list above is every single one of them made at least one All-NBA team.
When Barkley and Malone played, the quality at PF was just the opposite. Aside from the two of them, few PFs were noteworthy. When these two started playing in the 80s, most of the forward spots in All-NBA teams would go to SFs rather than PFs. SFs and F/Gs ruled the roost then with Erving, Bird, English, King, Nique, Worthy, Dantley, and later Pippen and Grant Hill, who were virtual locks for All-NBA and All-star berths in their respective primes. Some of the All-NBA teams either had 2 SFs (Bird/Nique, Bird/King) or 2 centers (Sampson/Kareem) or even 3 guards in the 2nd/3rd teams (Mullin/KJ/Stockton or Mullin/Drexler/Dumars).
Compare this to the modern era, when we have 3 PFs making the goddamn first team in 06-07 (Timmy, Dirk, Amare), 3 PFs making the 2nd team in 01-02 (KG, Webber, Dirk) and 07-08 (Timmy, Dirk, Amare). Also since TD came into the league, 8 All-NBA first teams out of a total of 11 feature 2 PFs and 0 SFs. That tells you a lot about the strength at the PF position.
So who did Barkley and Malone go up against? An aging McHale, Chambers, mings, Coleman, Detlef Schrempf, Kemp, Rodman, Juwan Howard, Anthony Mason, Vin Baker etc. The PFs of the modern era would eat these guys alive. Drafted in 1985, Karl Malone got to the 1st team only in 88-89. Drafted in 1984, Barkley made it to his first 1st team only in 87-88. Compare these to Duncan, who made it to the first team as a rookie in 97-98.
Barkley and Malone played mediocre defense. Duncan as a PF is one of the premier shot blockers in the NBA, and has been for the last dozen years. The Spurs' system revolves around him protecting the rim. Also, Malone was a jump shooter and screen/roll finisher while Barkley was a jump shooter who also drove on occasion. Duncan is the only low post offensive player out of the three. In the post-Jordan era, the low-post big man who also blocks shots has been instrumental in winning 8 out of 10 les. I don't know about you guys, but I'd take that over a jump shooter who can't intimidate at the rim any day.
The books don't lie. Duncan is the only player in NBA history to receive All-NBA and All-Defensive honors in his first eleven seasons.
With 4 les, 3 Finals MVPs, 2 MVPs, 11 All-NBA selections (9x First team), 11 All-Defense selections (8x First team), 11 All-Star game selections (10x starter), Duncan is the most complete player of the post-Jordan era and the best Forward ever.
And it isn't even ing close! You'll have to add up Barkley and Malone's achievements to top what Duncan has accomplished, either one of them pales in comparison. Even if Malone hadn't choked and had won a le he'd be behind Duncan career-wise, but the shriveled up in the clutch every single time! Even if Barkley had made it to a couple of All-Defensive First teams he wouldn't be in Duncan's class, but the dude never even made it to the second team once!
These guys aren't in the same sentence as Duncan. Bird or Dr.J would be a more interesting, but ultimately futile, argument. But these 2 underachievers don't hold a candle to Timmy.