Going off of
BubbleGamma's Guide to Defense Buff Numbers. I will try to share some info so you can make the best choice for your blaster build. (If you have more up to date numbers please share with us so I can update this page to help everyone get accurate numbers).
Stealth Powers
Stealth (Concealment power pool) (-0.38 EPS)
Invisibility (Concealment power pool) (-1.50 EPS)
Phase Shift (Concealment power pool) (-0.38 EPS)
Cloaking Device (Devices support secondary) (-0.30 EPS)
Smoke Grenade (Devices support secondary)
All Stealth powers have a base 7.5% to defense, (except for Smoke Grenade and Phase Shift). Each SO added to your stealth will give add 1.5 to your defense. Maxed out, it give 16.5% add to your defense. Stealth and Cloaking Device will allow you to attack your opponent, Invisibility & Phase Shift will not. Phase Shift is the ultimate damage mitigation power; you cant get hit when activated. Cloaking device does not suffer as much most stealth power due to not having a movement speed decrease and using less endurance compared to Stealth. Using Stealth will reduce your movement speed by 1/2. Most people counteract this buy taking Super Speed or Swift. (Stealth, Invisibility, Phase Shift, & Cloaking device can not be on at the same time. Example Stealth and Cloaking device cannot be use together)
Stealth & Agro
All 'stealth' powers have a -aggro. Mobs have a set perception, or aggro range. Stealth, Cloaking Device, and smiliar powers have a 'low' -aggro benefit. Thus a normal mob will be able to see you if you get too close. But you can penatrate its normal aggro range. What I mean is with Stealth and CD you can get closer to mobs before they see you, to a point. Invisbility, Grant Invisibility has a 'high' -aggro benefit. This is the size of a normal mobs aggro radius. For example, lets say a normal mob has a aggro range of 50. Stealth might only give a -25 beneftit to you. So when you came w/ in 25 ft the mob would see you. Invis and similar powers grant you a -50 benefit. Thus the mob can never see you, unless you aggro it. Again, these are just made up numbers to make a point. Snipers have an increased aggro range. In the above example, Snipers might have a 100 range. So Stealth and like power will only get you a bit closer, Invisibility wiill get you even closer. But you can't get close enough to never been seen. Super speed also has -aggro effect. You can stack these powers to increase you -aggro effect.
Smoke Grenades
Unlike other defense powers, smoke grenades work by hitting your opponent, and reducing the base accuracy of your opponent. Base is set to 10%. Unlike other defense powers it take 33% SO accuracy debuff enhancements; argo, with it fully slotted it will decrease your opponents accuracy by 30%.
Example:
An equal level minion has a 50% base accuracy to hit you.
Unslotted Smoke grenade will reduce his accuracy by 5% (10% of 50%), therefore an even level minion will have a 45% chance to hit you. Fully slotted it reduces to 35% chance to hit you [50% - (50% x 30%)]
Even level Lieutenants have a 62.5% base accuracy to hit you.
Unslotted Smoke grenade will reduce his accuracy by 6.25% (10 % of 62.5%), therefore an even level lieutenant will have a 56.25% to hit you. Fully slotted it reduces to 43.75% 5chance to hit you [62.5% - (62.5% x 30%)]
Even level Bosses have a 75% base accuracy to hit you.
Unslotted Smoke grenade will reduce his accuracy by 7.5% (10% of 75%), therefore an even level boss will have a 67.5% chance to hit you. Fully slotted it reduces to 52.5% chance to hit you [75% - (75% x 30%)]
So if your opponents base accuracy to hit is higher, your smoke grenade will have more of affect on your opponent.
Smoke grenades will help your team also.
Movement Powers
Combat Jumping (Leaping power pool) (-0.08 EPS)
Acrobatics (Leaping power pool) (-0.32 EPS)
Hover (Flight power pool) (-0.24 EPS)
Hasten (Speed power pool) (2.5 minutes duration, 20 endurance drain on termination)
Super Speed (Speed power pool) (-0.54)
Combat Jumping, Hover, and Hasten will give you a base 5% added to your defense. Each SO added to these movement powers will add 1% to your defense, which in turn 6 slotted gives you 11% add to your defense. Acrobatics will increase your resistance to knock backs and holds. (If anyone has the stats on how much please add it to this thread). Most people do not slot hasten with defense buffs but slot it with recharge buffs for perma-haste.
Super Speed is another stealth power that works the same as stealth powers above. It just doesn't get you any bonus to defense.
Other Power Pool Powers
Tough (Fighting power pool) (-0.20 EPS)
Weave (Fighting power pool) (Range & Melee) (-0.20 EPS)
Maneuvers (Leadership power pool) (-0.46 EPS)
Tough does not decrease your chances to get but reduce the amount of damage you get from the 2 most common types of damage, smashing & lethal. (Anyone have the stats for Tough?) Both weave and maneuvers have a base of 6.25% for blasters. Each SO added will add 1.25%, six slotted will give a total of 13.5% to added to your defense. Maneuvers uses more endurance but it works with your team and adds to your overall defense. Weave only adds to your melee and range defense. (Weave is actually higher for Tanks and Scrappers)
From the time of release to Issue 2 you are limited to 4 power pools.
Defense Caps
Remember, due to the cap, your opponents will have a minimum 1 out of 20 chance of hitting you (5% chance to hit). Maxed out to hit you is 19 out of 20 chance of hitting you (95% chance). Not including the streak breaker. YOU WILL GET HIT NO MATER WHAT!
Unequal Level Combat Modifiers to Opponent Accuracy
When you fight a Villain whose level is higher or lowers than yours, the accuracy and base defense is modified.
The modifiers listed affect your chance to hit or be hit.
Level / Modifier
+5 / x1.50
+4 / x1.44
+3 / x1.33
+2 / x1.22
+1 / x1.11
even / x1.00
-1 / x0.90
-2 / x0.81
-3 / x0.73
-4 / x0.66
-5 / x0.60
Example.
Fighting a +3 minnon his base to hit would be
50% x 1.33 (+3 level modifier) = 66.5% base chance to hit you.
Fighting a 2 lieutenant
62.5% x 0.81 (-2 level modifier) = 50.625% base chance to hit you.
Powers with Secondary Effects
Also, powers that knock down/back, stun/disorient, hold/sleep, or slow will decrease your chance of being hit. Knock down/back, and slow will reduce the speed of your opponents recharge and activation times, reducing their rate of attack. If stunned or hold, they cannot attack reduces their rate of attack to zero. Many blaster powers have these secondary effects.
How the Debuffs/Buffs work
A devices blaster has the following defense powers.

Smoke Grenades (6 slotted w/ accuracy debuffs)

Cloaking Device (4 slotted w/ defense buffs)

Combat Jumping (no slots for defense buffs)
Up against a +1 boss (75% x 1.11 = 83.25% base chance to hit), Cloaking Device (13% with the slots) and Combat Jumping (5%) he has 65.25% chance to be hit. His smoke grenade hit the boss (subtracts 24.975% because of the boss's base). Now he is at a 40.275% chance to hit. With all his defenses he reduce his chance to be hit by 48.37% against an +1 level boss.
Smoke grenades is still one of the best defense powers a blaster can get.
Enhancements
Certain SO enhancements will give you 33% or 20% increase on your powers. Here are the SO Enhancements that deal with defense powers

Defense Buff = 20%

To Hit Debuff = 33%

Resist Damage = 20%
You start buying most SO that you can use from NPC stores at level 22. You can only buy SO To hit buffs and Debuffs when you reach level 30 startng in Brickston and Founder Falls.
Other helpful defense links
BetaUserName's Defense Powers - Exact Numbers and Explanations
Vennom's Endurance Costs for Toggle Powers
RSRobinson's A powerleveler's guide to blasters (very long)
Notes:
09/20/2004 Initial post
09/20/2004 - Added helpful links "BetaUserName's Defense Powers - Exact Numbers and Explanations"
09/20/2004 - Added to helpful links "Vennom's Endurance Costs for Toggle Powers" (thx Key_Man)
09/20/2004 - Added BetaUserName's "Stealth & Agro" info and made some other changes in the stealth section.
09/20/2004 - Added to helpful links "RSRobinson's A powerleveler's guide to blasters" to
09/20/2004 - Added Super Speed to movement powers. Thx, Doctor42
09/20/2004 - Added Enhancement section.
09/20/2004 - Added info on stealth movement rate.
09/20/2004 - Added BubbleGamma's info on hasten under Movement Powers
10/17/2004 Added Endurance Drain Per Second stats
10/17/2004 Added Level Modifier change
10/17/2004 Updated and put in the guides forum
will try to keep this updated with any upto date information that you add to this thread that can be validated. I am open to any suggestions you may have to improve this guide.
Info I need for this guide:

The stats on Tough.

Defense list on Melee/Range/AoE/etc.. for each power
Special Thanks goes to BubbleGamma & BetaUserName (and anyone who participates in helping) for their assistance in this guide.