More of a real answer: I suspect that
Relentless is the weakest of the bunch, but he
is the easiest to cast, whereas Primal Hunter only goes in the heavy green deck. Then again, I'm running the big five mana lug, so take my word with plenty of salt.
Dom is probably right here, in that the two four mana options are optimal, despite clogging up that point on the curve.
In general, I wouldn't be averse to seeing more planeswalkers in your cube, especially if it helps out control. The
one Legacy control deck that I know of (there's probably plenty more, I don't follow the format) plays a grip full of walkers to help in stabilizing, and I think that's the missing piece in your cube's control decks. Having only removal, counters, and six-mana finishers isn't usually enough to get the job done, because some of those creatures keep coming back forever.