Wrath mentions that Lust has an Ultimate Spear that could pierce any substance.
Wrath also mentions that Greed has an impenetrable Ultimate Shield.
By definitions, Lust can break through anything. Impenetrable means that it is impossible to pass through or enter.
It's a case of Ultimate Attack vs Ultimate Defense.
Lust has the Ultimate Spear which can pierce through any substance. Therefore, it should be able to pierce through Greed's Ultimate Shield. Otherwise, if it can't, then it is not the Ultimate Spear.
Greed has the Ultimate Shield which is impenetrable. Therefore, it should be able to block Lust's Ultimate Spear. Otherwise, if it does not, then it is not the Ultimate Shield.
One of these have to crack. However, since we have no evidence to actually determine which is better, we can't make an educated judgement.
Therefore, if Lust's Ultimate Spear meets Greed's Ultimate Shield, then THE UNIVERSE WOULD EXPLODE!!!! THE UNIVERSE CAN'T COMPUTE A VICTORY SINCE IT WOULD CONTRADICT THE OTHER ULTIMATE WEAPON!!!
However, if Lust were to fight Greed without the Ultimate Shield covering his entire body, Lust would win. Let's face it. Greed is pretty dumb. Wrath defeated Greed because he cut him in the spots that were not protected. During the rematch between Greed and Wrath, Greed didn't learn from his past battle and only put up the Ultimate Shield up to his arms.
If Greed would have put on the entire Ultimate Shield from the start, Greed would have owned Wrath and Fu could have lived. That is why I would say Lust wins. Greed would be dumb and only put up his shield up to his arms, and then Lust would just stab/slice the parts that were unprotected.
Summary:
Lust(Ultimate Spear) vs Greed(Full Ultimate Shield): UNIVERSE EXPLODES!!!!
Lust(Ultimate Spear) vs Greed (No Ultimate Shield from the start): Lust wins.