With the new changes to the Mastery Trees, namely the massive buff the Defensive Tree receive, Renekton is very powerful with a 9/0/21 Mastery build. Here are the highlights:

Offensive Tree (9):
On Tier 1, take the improved summoner spells and the bonus attack damage. No reason to grab improved damage to minions.
On Tier 2, take the attack speed buff. I know it seems strange to grab Alacrity on Renekton, but these points are a means to an end.
Tier 3 is the prize. A static 10% Armor Penetration buff is fantastic. It really adds to Renekton's early game.

Defensive Tree (21):
On Tier 1, take the improved resistances. +6 MR and Armor is quite nice for 6 mastery points.
On Tier 2, take the health per level. I understand the desire to take the HP/5 mastery, but I prefer to save those points to take Initiator further down the tree. There is nothing wrong with taking points in Vigor as opposed to Initiator. That's a matter of play-style and personal preference.
on Tier 3, take the static bonus health and the reduced AoE damage. I don't see any reason to take the static reduction from Indomitable. Yes, it is nice, but I would rather take the reduced AoE percentage damage. 3% or AoE's is going to be more more than what Indomitable provides. However, placing points in Indomitable as opposed to saving those points for Initiator makes perfect sense as well. My play style is aggressive and I really enjoy extra move speed. If you prefer more survivability, go that route.
On Tier 4, I only take Initiator. As I have already said, I love move speed and I love starting fights. If that isn't in keeping with your play-style, take these three points and put them into Vigor or Indomitable, whichever you prefer. I don't see any reason to take the CDR on this tier. It is so insignificant that the points are better spent elsewhere. As for Siege Commander, I don't find it to be overwhelmingly powerful, but if you prefer it over Indomitable, by all mans take it.
On Tier 5, take Honor Guard. The Mercenary talent is neat, but I don't find it to be that valuable. The math on it just doesn't add up for me.
And of course, the end goal is the Tier 6 Mastery. What an amazing mastery. 10% reduced CC.