Which one you should focus on is definitely a matter of personal preference. Each person who suggests something brings their own inherent bias against certain commanders/strategies and for other certain commanders/strategies.
In an effort to avoid being biased, I'll just discuss what I think the role these decks would play & their power level. Obviously, they won't be perfect and my opinion might change depending on certain inclusions. Whichever one appeals to you the most might be the best option.
Archelos Group Hug: Very politicky (definitely a word, nothing to see here) & more on the casual end. You mentioned "speed control," so if you intend to handicap certain players who are getting ahead, this deck could also inspire some salt.
Kathril, Aspect Warper: I've built this, and it's very hit or miss. Either you hit key tutor cards like
Buried Alive,
Fauna Shaman, etc. or get lucky with self-mill effects and get a crazy good Kathril that threatens the game as a whole
or you don't necessary cards and durdle/your opponents hit you with grave hate or a removal spell on Kathril. Lot of variance, which could be refreshing. Probably a bit higher than Archelos Group Hug in terms of strict power level
Sen Triplets: I'm not quite sure how Rogues & Mill play into Triplets, but I will say that Triplets when paired with a good 5 color mana base (to play your opponents' red & green spells) can be incredibly frustrating to play against. It is strong (probably more-so than the previous 2 commanders) and very salt-inducing.
Tariel, Reckoner of Souls //
Obosh, the Preypiercer: This seems super spicy. One major aspect of this would be the variance, much like Kathril. Being limited to odd CMC spells and being very reliant on your opponents having good creatures in the graveyard + you getting lucky with the random choice means that it will be wildly hit or miss. Now, I think that (depending on your exact list) it can be less hit or miss in some aspects than Kathril because your list isn't going to necessarily be centered around Tariel. Probably between Archelos & Kathril in power level mainly because of the odd CMC only restriction & Tariel's randomness.
Xyris, the Writhing Storm: I've had the
misfortune to play against several Xyris wheel decks and they can be pretty insane. Wheel a few times, make an absurd number of snakes, win with a pump spell,
Impact Tremors,
pick your poison. Power level would probably be right around
Sen Triplets, with each taking the crown depending on exact lists.
Hopefully this was helpful!