I'd say SD and Elec Armor would be best if you want to build on the burst AoE strength of Elec Melee. SD gives you AAO and Shield Charge which both give a boost in AoE burst damage. Elec Armor gives you a recharge bonus, so BU and LR are up faster.
In terms of overall synergy however, which primary would be best is very debatable.
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I would say Stone Armor, if only because LR is a second teleport.
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Didn't expect the first 4 responses to be 4 different answers. Leaning towards Elec Armor, might even go Scrapper for SR.
Tales of Judgment. Also here, instead of that other place.
good luck D.B.B.
While not being perfect I would think Elec/ would be a nice pairing due to the end draining capabilities it has that could be leveraged while herding.
I'll tell you more about that as I level mine up. 
Well, I'm currently working on two /electric tankers, one an Ice/Elec at around level 30 and one a Dark/Elec at level 25. At this point the Dark/Elec may have a slight edge but I see the Ice getting really good once it matures and I get some sets into it.
Both tankers seem pretty capable as far as I've taken them, when I get time to play them the rest of the way I'll have a better idea. Ever since i19 hit I've been playing my 50's and haven't touched the lowbies
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How about thematic considerations? I paired electric melee with electric armor. Loads of fun. Took him to 50.
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In terms of taking advantage of what Elec Melee has to offer, what's the best Primary to pair with it for a Tank for PVE?
Tales of Judgment. Also here, instead of that other place.
good luck D.B.B.