Leth, no contest. Ek is more durable and more tactical, but that doesn’t make much difference in the face of Leth’s overwhelming lightning onslaught. If they were in an all-out one-on-one fight, it would be over in about five seconds, and Ek would be ash.
Given time to prepare a trap, Ek might be able to rig up grenades and get the drop on Leth. But a situation like that most likely involves military assignment and his entire squad, so that would be cheating as far as this prompt is concerned.
Interestingly, they did actually meet on the battlefield once. An imperial intelligence minder had just activated Lethen’s obedience programming in a foxhole, but died to shrapnel before he could give an order. Leth sat there, a silent blank slate awaiting commands… until pub trooper Ekhein, lost behind enemy lines, dove into the foxhole for cover. He yelped in surprise to find a big-eyed, skinny-framed noodle boy waiting there in silence. Leth tended to wear very simple robes as Lord Vereri, so Ek thought he was a civilian. All Ek said was, “Stick with me, buddy, we’ll look out for each other!”
This worked for a while (Leth following silently behind Ek, who assumed he was regular shocked and afraid, not brainwashed-sith shocked and afraid). Ek was very affectionate and comforting to Leth, patching up a few scrapes, ruffling his hair, and offering to help Leth reunite with his family. Leth never responded verbally, though he did smile a few times. Eventually, however, a huge battalion of Imps was sent to find the missing sith lord. They found the two men a few miles from the republic lines, but when they moved to shoot Ek, Leth killed them all. With force lightning.
Leth, speaking for the first time: Looking out for the buddy.
Realizing that the best way to “look out for” Ek was to disobey the “stick with me” part of the order, Leth pointed Ek to the republic encampment, returned to the imperial army, and promptly had all his memories of Ekhein erased by imperial intelligence.