After approximately five seconds of searching ground types with high stat totals, I can tell you that Rhyperior is a great counter to Regieleki for multiple reasons.
1. Electric immunity.
2. Normal, flying (acrobatics), and rock (ancient power) resistance.
3. Access to Brick Break, destroying all screens (Smogon’s choice set right now).
Basically, Regieleki can’t do anything to this dude, so Rhyperior is free to set up (swords dance!) or put up stealth rock. However, Steelix is a close pick due to its Mega being used in UU and similar typing, but no access to Brick Break.
EDIT: This is not taking competitive rank into consideration, thanks Fizz for the comment.
EDIT 2: Everything following this sentence, since I thought it would be appropriate to accommodate SSWD’s actual question, which is what Pokemon can wall Regieleki IN UBERS.
Alright, second time’s the charm? Regieleki AKA Speed Demon is known for its speed (duh) and attacking stats at 100 each. However, its defensive stats are severely lacking for a legend (50 each) and 80 base HP. Glass cannon, anyone?
99% of this thing’s moveset is electric, with Electro Ball being a standout due to the (duh) high speed. Obviously, we should take a ground type for this beast.
My Pick: Zygarde, AKA Slithering Ecosystem. The Dragon-Ground type is immune to Electric moves, access to Brick Break (Smogon’s set of choice for now is setting up screens), setting up with Dragon Dance and/or Substitute as Regieleki has no chance of doing anything offensively, and that it’s an Uber. Need I say more? Plus, Power Construct is awesome. Free HP bulk, especially after two substituted, is never unappreciated.
Honestly, there aren’t many more reasons I can list without a list of your team members. For example, a Garchomp on your team would obviously run this whole thing to the ground. I hate ice as much as the next person, but two 4x weaknesses is WAY too much. Overall, Zygarde with its Power Construct ability (substitute!), placement in Ubers, ground type, brick break, and great bulk help it get maybe one or two free turns when switching it into Speed Demon. Jeez, my thumbs hurt from typing so fast…