These are my results with various gems in a lightning WW build. Test subject was Gom, each gem was tested multiple times with multiple gear setups, this particular set I'm posting was done with Schaefer's hammer with WW+CotA and no WotB (using WotB kept the results well within a 0.05 margin of error, though). Mileage will vary based on your stats, but the ratio's amongst each other for WW should hold up nicely:
1. Mirinae (r26) is #1 at a 37% DPS increase vs single target. Against 2 targets it is cut in half to 18.5% and falls behind BotTrapped, against 3 it is divided by 3 falls to 12.3% behind BotPowerful and Pain Enhancer, against 4 it drops to dead last. This is tied for my #3 gem slot, depends if I need more RG killing power because aside from RGs and the occasional elite, we should be ignoring single targets. You get the most out of this gem from stacking Elite damage.
2. Bane of the Trapped (r27) came in at #2 for single target and lightyears ahead of the rest for multi-target. It is providing the exact DPS boost that it lists (23% in my case). This will be taking my #1 gem slot.
3. Pain Enhancer (r25) came in 3rd for Single target and second for multi-target at 16% DPS. The DoT scales well and the gem gets stronger for multi-target with the r25, combined with Rend this should skyrocket for AoE DPS getting close to (or possibly surpassing) BotT. This will be taking my #2 gem slot.
4. Bane of the Powerful (r25) elite damage passive only was a 10.5% DPS increase. If the Proc buff never falls it should be close to 26.5% vs elites and 16% vs trash. My stats with 25% Skill dmg and 42% elite dmg before the gem buffs are important to note: the Elite damage is additive with Elite damage (obviously), while the 20% damage proc is counted as "Skill Damage" making it Additive with things like Yan's pants and Battle Rage. The more you have in each category the more it devalues this gem in terms of damage so you will have to figure it out for your stats. It has higher potential than most gems assuming Elites are perfectly spaced to maintain the buff. Expires at death, on RGs, and on long waits between Elite packs. Potency never increases, leaving room for other gems to surpass it at high ranks. This will only get my #3 slot if I find the uptime satisfactory (so far it is not).
5. Wreath of Lightning (r25) was an 11% DPS increase. Has solid potential on large groups, but r25 does nothing for damage leaving it in the dust. Despite gaining my % Lightning bonus, it is not keeping up with the rest. I like this as a R1 to use as a placeholder until my BotT is R25 and ready to be socketed. Even though it benefitted from my Bonus %Lightning damage it still couldn't keep up with Pain Enhancer (kind of disappointing)
6. Taeguk... I didn't test it this time around since mine is only lvl 3, and its complete crap at lvl 3. Scales better than any other gem, so the potential damage is higher than anything except BotTrapped. Pain in the !@# to maintain, would have to play with it for a while to see if its worth the trouble. It counts as Skill bonus damage, so at r25 the damage boost for me would be 17% (expected range is 15-19% based on your gear/skills), but you would also gain 23% armor (the actual damage reduction is much more difficult to calculate w/ diminishing returns).
Hope this answers some questions about gem choices. Thanks for taking the time to test all of these things and posting the results, Juggernaut. Really good stuff to share with your fellow Barbarians.