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There are couple of points to be made regarding this:
- Buggy's words do not hold much value.
- Luffy doesn't look tough does he? But if his friends are cornered, then Luffy becomes a different monster altogether. It was said that if Roger's crew was put at risk, nothing could Roger. This means that Roger's best can only be expected when his crew is in danger. And guess what, we have never seen Roger's crew in REAL danger. So, we've never seen Roger's best. Against WB, Roger's best could never appear because WB (due to his good nature) doesn't pose any REAL danger for Roger's crew.
- Roger's time didn't have the concept of Yonkou. If it did, Roger would surely have to surpass them. Meaning he would have to better than other pirates. Garp was the strongest marine at the time and Roger was able to deal with him.
- If Xebec was alive, do you think Roger would be able to reach Laugh Tale? Answer is NO. Roger defeated Xebec, which allowed him a clear path to Laugh Tale.
- If anybody else tried to reach Laugh Tale, would he/she be able to do it without defeating Roger and dealing with Garp? Again, the answer is No. Roger would've got in the way.