Manny Pacquaio is not the best athlete of the last decade. He's an exciting fighter to watch, but he is not the best boxer in the world at the moment (far too many flaws in his style). He's been very fortunate over his last few fights to fight either severely weight drained fighters (De la Hoya didn't rehydrate after weighing in at 147; he was on an IV just before the fight--Manny actually out weighed him) or come-forward fighters who make him seem like a giant killer (Hatton, Cotto). Not to mention that he's been beaten once in this decade Morales) and got a bit of a gift decision that spared him a second loss (Marquez). I'm a big fan of his, but as I said he's not even the best boxer in the world--which makes it hard to make him the best athlete of the decade.
So who's better than Pac-man? I imagine you'll argue Mayweather, and I'll agree that Mayweather is more skilled (in fact I'm heavily leaning Mayweather in their fight) but Pretty Boy Floyd simply hasn't proved he's the best in the ring. He beat some great guys in Corrales and Gennaro Hernadez in his early days (which is how he rose to be #1 P4P) but he arguably lost to Castillo in their first fight, and ever since the Castillo fights he's not fought truly challenging opponents. He didn't fight Acelino Freitas, he didn't fight Joel Casamayor, he didn't fight Paul Spadafora, he didn't fight Kostya Tszyu, all fights that could've been made and all fights he would've probably won. But you said it yourself somewhere else, to be the best you gotta beat the best.
And his competition since he's been a welterweight has been vomitous. Sharmba Mitchell, Zab Judah, Juan Carlos Baldomir? He didn't fight Margarito, even though the fight was thrown in his lap, he didn't fight the likes of Cotto, or Mosley. Credit for moving up to light-middle to face DLH, but that's not the type of fight that your legacy should rest on.
If he'd beaten but two or three of the guys I'd mentioned, I don't think I could argue against you, but Mayweather simply hasn't proven himself against worthy opposition. Which the Pac-Monster has. He's beaten a host of top guys, at numerous weight divisions. And who cares if he lost to Morales once, he went on to win twice. Noone cares about the first loss if you revenge yourself, especially twice. I don't care about one loss if it came to a great guy. If Pretty Boy Floyd had fought Acelino Freitas, Joel Casamayor, Leonard Dorin, Paul Spadafora, Stevie Johnston, Kostya Tszyu, Miguel Cotto, Antonio Margarito and Shane Mosley noone would care if he lost to one of those guys if he beat all the others. He would be the undeniable greatest fighter since Ali, maybe even Sugar Ray Robinson. But he did not.
Mayweather has everything you'd need to become one of the all time greats, except the record.