9/21/0 , 0/21/9 , 0/25/5 , 1/25/4 are almost the same.
However I believe that 9/21/0 has the best trade offs.
For 'Default Singed' Masteries ( or )
Offense Tree (9)
Double Edge Sword gives you 2% extra damage but also makes you take 1% extra damage. The net value of this mastery is 1% if you would fight someone and both of you die. This mastery is a little tricky but it is usually in your favor
2.5% Cooldown Reduction is preferred over Attack Speed as it scales throughout the game
Feast Mastery effectively gives you 2hp5 and 1mp5 if you manage to kill 6 minions every 30 seconds
0.89AP/lvl slightly increases your damage. There is nothing better to take when you are climbing this tree
8 AP ncreases your Poison damage by 11% at l evel 1 (22 -> 24.4)
Defense Tree (21)
Block blocks Auto Attack damage by 2 before Armor reduction is taken into account (This means 100 damage is reduced to 98 and 100 armor will further reduce it down to 49)
5% Bonus Armor/Magic Resist gives you about 1 armor at level 1 and more throughout the game. When you have 80 base armor and 150 bonus armor, you get 7.5 extra armor which makes you take 2.22% less damage against physical attacks (100 + 230 / 100 + 237.5)
36 HP increases your MAX HP by 6.5% at level 1 (assuming 550 HP)
Unyielding blocks any damage (except true damage) after Armor/Magic Resist reduction is taken into account (This means 100 damage can be reduced to 50 with 100 armor and down to 48 with 2 points in unyielding)
3.0% HP is good on Singed since he tends to stack HP items (but not as much with )
Flat Armor/MR reduces your incoming damage by 3.7% (Assuming that you took 5 Armor and 30 Base Armor)
4 Armor 2 Magic Resist per nearby enemy champion reduces incoming physical damage by 2.3% x Champions and magical damage by 1.4% x Champions (Calculated with 70 Armor and 40 Magic Resist)
3% Damage Reduction is like negating 1 attack in every 33 attacks. This is very good for a 1 point mastery. It works very well with Rylai's Crystal Scepter
Regaining 3% of your missing health every 5s is the best mastery in this game. Notice how the stats of this mastery is not added on to your health regeneration stat but rather heals you every 5s. For example, if you're missing 2000 health, this mastery will suddenly heal you for 60hp and again after 5s with reevaluated missing health.
15% Tenacity is a useful mastery which stacks multiplicatively (this means there's no diminishing returns nor increasing returns when tenacity items are stacked) with
There is no need to get 10% Summoner Cooldown Reduction (Master Mind) because you are not always going to gank other lanes constantly with
If you ever run out of mana, you recall. You will not get enough mana for fling from 3 mana regen/5.
10% Crit Block is useful late game as half of the damage that AD carries deal are Critical Strikes. Usually, you receive about half of the total damage from AD Carry so we can assume that this mastery only gives you 2.5% damage reduction late game. Early and Mid game, it does not do anything so it's better to spend that point for extra armor early.
10% slow reduction (Relentless) is not that good of a mastery. Consider yourself having 400 Movement Speed and you've been slowed with 40% slow (400->240). With this mastery, you'll be slowed with 36% slow (400->256). Congratulations, you'll move 6.7% faster when you're slowed with