I dunno, Magruder was my favorite general in the west... not that he made the list.
Not that there ever was a sizable force from either army out west.
And Texas was never really conquered!
Magruder was assigned to the Trans-Mississippi but he never accepted the command and though he did command a force out there it was never an Army.
Benjamin McCulloch and Sterling Price both commanded small Armies early on that fought in Missouri and Arkansas before they were combined into the Army of the West under Van Dorn who took that force across the Mississippi after losing at Pea Ridge.
Thomas C. Hindman was building an Army in Arkansas by hijacking troops being sent east and actually managed to drive a Federal force out of Arkansas before he fell foul of his political opponent.
Holmes commanded a force of at least 10,000 around the time of the Vicksburg campaign, which was about as big as any Confederate Army got West of the Mississippi, but then agains Holmes was a terrible theater commander and administrator and who knows how many men he could have had. Joe Johnston estimated he could have had theoretically around 50,000 but that seems a bit of a wild estimate to me.
Dick Taylor famously commanded an Army in Louisiana and during the Red River Campaign.
John George Walker commanded a small Army against Frederick Steele in the "Camden Expodition" and drove him to retreat.
All in all the Trans-Mississippi is a tale of neglect from Richmond and a lot of untapped potential in the Generals, particularly in Taylor, Walker, McCulloch and (though he's not on the list) Joseph O. Shelby.