This is an analysis of how useful different masteries are.

Keep in mind that even though my knowledge of the game is vast, I'm only stating my opinion.

The builds are just some examples of what masteries I'd use on some champions. I play most of them, but not all.

AD = Attack Damage
AS = Attack Speed
CrC = Critical-strike Chance
CrD = Critical-strike Damage
ArP = Armor Penetration
MrP = Magic/MR Penetration
HP = Hit/Health Points
HP5 = Heath-regen Per 5 seconds
MP5 = Mana-regen Per 5 seconds
CDR = Cooldown Reduction
AP = Ability Power
MR = Magic Resist
GP10 = Gold per 10 seconds
/lvl = Per Level (bonuses which you gain a part of with each level)
AoE = Area of Effect

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Overall analysis of the Offense tree

Currently the overall strongest mastery tree because of how much every point in the tree is worth. The points in defense and especially utility are worth less.

You could run 21 utility or defense and go 9 points into offense for 10%ArP or MrP, but right now it's just not worth it. RIOT, BALANCE MASTERIES PLOX.

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Overall analysis of the Defense tree

It contains a few stats that are attractive to tanks and tanky DPS, but the general value of the masteries are slightly lower than the ones in offense and I'd like to see a few of them buffed.

Unless your champion is very mana hungry you should probably be spending 9 points to reach the 9-point-wonder Veteran's Scars on most champions.

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Overall analysis of the Utility tree

The worst mastery tree right now simply because the masteries in tier 3 and 4 are BAAAD (with the exception of Runic Affinity ).
Most other masteries are pretty good, but if you have to spend 8 points on bad masteries, it's sooo rarely worth it to go more than 9points into utility.

It's a good idea to go 8 or 9 points in if you're playing a mana hungry champion or someone who has great use of Runic Affinity or Swiftness .

I'm using only basic items to calculate cost effectiveness.
I'm counting PER POINT, not with all the points.

"Approximates" means that I had to use an item with more than one stat to calculate the gold effectiveness. I can't get an exact number there. The numbers on those are probably slightly off, but shouldn't be more than 5 gold off (which is a fairly small number).

Offense

Brute Force - 41.5g
Mental Force - 21.75g <-- OMG RIOT >.>
Alacrity - 28g <-- OMG RIOT >.>
Vampirism - 45g

Archmage - 27.2g per 100AP (this is just in the "twenties" because I put an even number like 100 there. It's not low)

Approximates:
Sorcery - 37.5g

Defense

Resistance - 33.33g
Hardiness - 33.33g
Vigor - 29g
Veteran's Scars - 79.17g

Mercenary - 8g per kill and assist

Utility

Meditation - 55.7g
Wealth - 20g <-- OMG RIOT >.>

Greed - 3g/minute 70.5g@25min (90sec at the start goes without gold income)
Strength of Spirit - ~40g per 400mana you have

Approximates:
Transmutation - 28g <-- OMG RIOT >.>
Intelligence - 75g

Per level masteries

Deadliness -
Blast - 5.44g/lvl 98g@lvl18
Durability - 3.96g/lvl 71.25@lvl18
Expanded Mind - 8g/lvl 144@lvl18

Approximates:
Enlightenment - 5.6g/lvl 101g@lvl18


All in all the big problems seem very clear here. Any mastery in the "twenties" (worth below 30gold per point) needs a buff really badly.

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Honor Guard and Havoc

They can't be measured precisely, but if I compare them to similar stats I can get a relatively good figure of what they're worth.

Havoc compared to AD:
1AD=41.5g
Damage = 50 = Havoc is worth ~10.375g (1.005*50-50=0.25damage)
Damage = 100 = Havoc is worth ~20.75g (1.005*100-100=0.5damage)
Damage = 200 = Havoc is worth ~41.5g (1.005*200-200=1damage)
Damage = 400 = Havoc is worth ~83g (1.005*400-400=2damage)

Not too shabby numbers I must say.

Honor Guard compared to armor/MR:
1Armor/MR=16.67g
Armor/MR = 25 = Honor Guard is worth ~20.94g ((100+25)/0.995-125=0.628armor AND MR)
Armor/MR = 50 = Honor Guard is worth ~25.1g ((100+50)/0.995-150=0.754armor AND MR)
Armor/MR = 100 = Honor Guard is worth ~33.5g ((100+100)/0.995-200=1armor AND MR)
Armor/MR = 200 = Honor Guard is worth ~50.26g ((100+200)/0.995-300=1.51armor AND MR)

So you need more than 100armor and MR to make Honor Guard about as good as Resistance and Hardiness .
WAY too weak for a tier 5 mastery.

Why is it that Honor Guard is so much worse than Havoc then?
That is because damage reduction has diminishing returns. ANOTHER case of Riot grouping totally different stats together --.--

Riot seriously needs to buff some masteries, because the balance is way off right now.

These NEED buffs:

Mental Force - Buff to 2 AP per point.
Vigor - Buff to 2HP5 per point.
Transmutation - Buff to 1.5% Spell Vamp per point. (increase point total to 4 for an even number?)
Wealth - Buff to 50gold per point. (but if Greed isn't needed 30gold will do just fine)
The above just can't compare to good masteries if these buffs are not implemented.

These could use buffs, but aren't complete **** (like the ones above):

Summoner's Wrath - Buff the Ignite part to 10AP and 5AD.
Tough Skin - Buff to 2 damage blocked vs minions/monsters per point.
Indomitable - Buff to 1.5 damage blocked per point.
Evasion - Buffed to 3% reduced damage taken from AoE abilities per point.
Honor Guard - Buffed to 1% reduced damage taken per point.
Scout - Buff to +10% increased vision ward sight.
Greed - Buff to about 0.8 per point. (or even 1 per point, but that may be too strong. 1GP10 is more balanced than 0.5 in any case)

Less important balance changes I believe would be good:

Executioner - Buff to activate at below 50% HP.
Durability - (ONLY IF Veteran's Scars is nerfed to 25HP) Buff to 2HP per level and point.
Mercenary - Buff to 10gold per kill and point.
Juggernaut - Buff to +5%HP.
Sage - Buff to 50exp per kill.

I implore you, Riot. Please balance masteries while the iron is still hot.

Keep in mind that these are just the most standard masteries. There will be champions who don't fit my suggestions.

Casters: 21/0/9

You reach the mana, mana-reg and buff duration with just 9 points, then you can spend the rest on power. Until Riot buffs the utility masteries offense is simply a better choice for anyone who scale well with AP.
In some rare cases you might benefit from 9defense more than utility, but you are a caster after all and mana is almost always very useful.

Support casters: 0/9/21

While utility is very lacking right now, supports just don't benefit much from any other masteries.

Crit/AD carries: 21/9/0

Simply benefits most AD carries more than any other masteries. Sure you could go 9 into utility, but you need to consider how much laning power you'll lose, because 9 defense helps a ton.

Physical damage tanks/offtanks: 21/9/0 or 1/21/8

Pick defense or offense depending on your needs.

Magical damage tanks/offtanks: 1/21/8 or 21/9/0

Pick defense or offense depending on your needs.

Physical damage junglers: 21/0/9

Offense is simply more powerful than other masteries. Use it.

Magical damage tanky junglers: 0/21/9

Bladed Armor just helps a ton for these junglers.

Manaless champions: 21/9/0

Most of them should be running this setup because chances are they won't have much use of utility.
Offense gives the most bang for your points right now, so if you have the choice; chances are that you'll want offense.