His run above LW:
2-1 vs Hughes (Hughes is 2nd GOAT WW)
Draw vs Fitch (Fitch at the time ranked #2 WW)
1-0 vs Ludwig
2-0 vs MWs (Rodrigo & Renzo)
0-1 vs Machida (eventual UFC LHW champion, Machida was undefeated at the time, Penn lost decision)
0-1 vs Diaz (top 3-5 WW at the time, decision)
0-1 vs Rory (past his prime, Rory was top 3-5 WW at the time, decision)
0-2 vs GSP (GSP is the GOAT, Penn lost 1st fight as split decision)
There is no LW on the planet who would've put up as competitive a fight vs Machida, who was a HW for that fight. Also, every WW on the planet loses to GSP, no shame in that at all. The Rory fight was Penn's only bad fight outside 155, though Rory was & still is regarded by many as a elite WW. Penn actually won 1st rd vs Diaz, so he lost but it wasn't embarassing. Context is needed. Despite taking 12 fights a WW or above, he was only finished twice, by the 2 GOAT WW in GSP & Hughes (whom Penn has a winning record against)