Illusion will solo the best of those three mentioned due to Phantom Army and Spectral Wounds. There is some minor duplication in the stealth powers. And you'll want a KBtoD IO in Phantasm to keep him from blowing mobs out of your Tar Patch or sending your Darkest Night anchoring sailing.
Ice also has a bit of duplication in the patch area with Ice Slick and Tar Patch, but it's only in secondary elements. Specifically the -Spd and ability to bunch up spawns. Ice Slick's primary value is the KD. Tar Patch is mostly for the -Res. One other small drawback is the need to turn off Arctic Air if you want to get significant value out of the Stealth element of Shadow Fall. Other than that it's a solid combo in the traditional control line.
Electric, like Illusion, is not a typical control set. It has several ways to reduce foe damage: the classic AoE Hold, a ground-targeted Sleep AoE that reapplies periodically, a chain Confuse, a chain KD and simply draining the END of foes flat with Static Field, Conductive Aura and Jolting Chain. The latter generally doesn't work well on fast moving teams. Nor is the sleep all that helpful in such situations. Which leaves Elect tending to under-perform most other sets in fast-moving team play in exchange for it's versatile and unusual set of control powers. Personally, I think Elect works best with Kin for the added END drain and the highly sought-after team buffs to offset the fast-paced play short-comings. I'd put this one behind the other two as a combo for /Dark.
Briefly on the other options:
Fire has a decent synergy with Tar Patch and Hot Feet. Comparable to Ice but with damage in place of slow and Stun in place of Sleep and Confuse. Earth duplicates the -Spd and bunching with all of Quicksand, Earthquake and Volcanic Gasses. But, once again, it's in the secondary element of -Spd and clumping. Dark has more -ToHit to stack on foes, but uses cones to do it (Fearsome Stare and Living Shadows) so is more of a range control set.
Mind and Grav have better single target damage due to Mesmerize/Levitate and Lift/Propel. Mind has a tricky Hold toggle that repels things. Sometimes useful, sometimes counter-productive. Otherwise it relies on an AoE Confuse and the standard AoE Hold with Sleep and Fear to back those up. Grav also has an AoE Hold, a teleport Stun that unfortunately also has a KB (which I believe you can downgrade to KD with the IO). Like TK it can be tricky to use, but to a much lesser degree (especially if the IO works, which it should) and is a better overall power. Grav's last AoE control is also tricky to use: Dimension Shift. That is a targeted AoE toggle that essentially creates a pocket dimension where those in it cannot affect those outside and vice versa. It's much a better power than /Dark's Black Hole. Both of these sets will solo better than most due to the single target damage. Grav's pet is also ranged and so better behaved than most. And stacks Hold for added control.
That leaves Plant. Like Dark more of a ranged control set due to it's bread-and-butter being a cone. But it's a lead-off cone, so not a big deal if you want to be in melee range thereafter. You have the AoE Hold and an AoE Sleep but otherwise it's atypical. There's the aforementioned cone, Seeds of Confusion, which is as advertised a Confuse and highly effective (at the cost of a bit of XP/mob, though not XP/min). And Carrion Creepers, which a location targeted oddball Slow/-Fly/-Jump power that does a bit of damage and KD which gets more common as you defeat foes and the creepers proliferate. You've also got a location +Reg power in place of another control. Though Spirit Tree can be used in inventive ways (it used to be able to block doors or take alpha strikes, but I haven't tried that out in Homecoming). Fully slotted it give close to +300% Regen, which coupled with Soul Absorption can boost team Regen greatly or alternated make sure the team has a good +Regen boost most of the time. Roots, the AoE Immob, does more damage than most as there is no secondary effect and is often slotted as an attack. My last point on Plant/Dark is that your best Plant power is an AoE Confuse, which makes Darkest Night less valuable.
