Well, OK, good points. Still...
Boast Busters:Spike, presumably knowledgeable as Twilight's secretary, points out that other unicorns are generally fewer-trick ponies, than Twilight's 25. (Trixie had, oh, about five. She's also probably higher than normal, or at least good at adapting what she knows to outshine each town's brightest.) Twilight's talent IS picking up others' talents/new spells/being magical, remember. This implies that it'd be at least difficult for other unicorns. (insufficient data, really. Maybe Pinkie Sense can find 'em too.)
I think I'll have to go with parents of Diamond Tiara/Silver Spoon then as "richest Ponyvillains".