Epic for Electric/Shield Scrapper
You want Blaze for fireball and to a lesser extent fire blast.
Grant Cover is actually quite a good power. It gives you some of the Defense Debuff resistance you'll need as Shield - it helps mitigate -Def debuffs from being hit by bullets/swords. Also, your team will appreciate the huge defensive buff it provides. It's a good mule for a LOTG 7.5% enhancement at that.
I would say either the Fire Pool or the Mu Patron pool are good. With Mu you can get a single target blast (Mu Bolts) and Ball Lightning one right after another. With the fire pool though you'll need to grab 2 powers before you can get Fireball. It's a matter of taste, both pools are quite good in my opinion. It's a matter of preference.
@Kalen on Virtue
Worth considering is how you plan on working your rotation out. If you want fireball to be your end-of-the-spawn hit, it might be just as good as Ball Lightning. Aside from the DoT, they're the same in total damage.
I went with soul mastery and took dark blast and shadow meld on my elec/shield.
50s: Inv/SS PB Emp/Dark Grav/FF DM/Regen TA/A Sonic/Elec MA/Regen Fire/Kin Sonic/Rad Ice/Kin Crab Fire/Cold NW Merc/Dark Emp/Sonic Rad/Psy Emp/Ice WP/DB FA/SM
Overlord of Dream Team and Nightmare Squad
It's also great for incarnate trials even heavily IO'd.
50s: Inv/SS PB Emp/Dark Grav/FF DM/Regen TA/A Sonic/Elec MA/Regen Fire/Kin Sonic/Rad Ice/Kin Crab Fire/Cold NW Merc/Dark Emp/Sonic Rad/Psy Emp/Ice WP/DB FA/SM
Overlord of Dream Team and Nightmare Squad
Mine is fire. I got no complaints, even if I do't use the hold much it slots 4 pcs of Basilisk's Gaze and I can't complain about that. And both fireball and fire blast are very useful, it's not like I've got a surplus of single-target attacks to finish off bosses with. ^_^

my lil RWZ Challenge vid
On my elec/elec I went with the fire epic, but that is because I have zero end issues, being /elec. With /shield you may run into problems, unless you have a lot of +recovery from other sources. On scrappers without infinite end, I tend to go with body mastery.
Hi Folks,
I'm trying to figure out the best epic for electric / shield. Is it the blaze (fire) chain? That probably does the most damage. However, thinking ahead, the chain with conserve power (perfect body?) may be better because it gives 2 endurance mgmt plus torrent which is another knockdown and will allow for the Spiritual Alpha. Also, is grant cover any good? I appreciate any input! Thanks.