Like most Pokmon cards, the value of Umbreon depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (XY Black Star Promos)
Currently, tier one of Platinum is the highest, and there is no indication of a special rank for players who are high on the leaderboard
Aside from synergizing with its Commander deck's featured commander card, it can also work with other commanders counting +1/+1 counters, such as Animar, Soul of Elements
Blitzwing gains that ability until end of turn
As for Runehorn Hellkite, I've never met a Wheel of Fortune I didn't like, and even though this one is pretty expensive at six mana, it also generates random value from the graveyard if you discard it to something like Survival of the Fittest and is uncounterable and instant speed