National Dex RU has the following Megas: Houndoom, Steelix, Sceptile, Ampharos, Camerupt, Glalie, Abomasnow, Audino, and Banette. Mega Ampharos, Abomasnow, Audino, and Banette are UR on NatDex RU's Viabilty Rankings (though Mega Ampharos is decent). I asked on the NatDex RU thread, and the response I got was Mega Houndoom. I'll be going over the top 3 Megas. If you want an answer including all available Megas in NatDex RU, you can read this post. The top 3 is based on what rank they are in the VR. (Click Resources, then Viability Rankings on this post.)
Mega Houndoom
Mega Houndoom is more likely than not the best Mega in NatDex RU. Having a solid speed tier, Nasty Plot to boost its already high Special Attack, Dark Pulse flinches to cheese switch-ins , and access to coverage moves such as Hidden Power Grass and Sludge Bomb make it a good wallbreaker that leaves only a few Pokemon such as Diancie and Milotic that can reliably switch into this fiery he11hound. Mega Houndoom is currently A+ on the NatDex RU VR.
Mega Steelix
Mega Steelix is A- on the NatDex RU VR. Mega Steelix has some traits that set it apart from other defensive Steel-types such as Registeel and Bronzong. Unlike the mentioned Steel-types, Mega Steelix isn't passive and has an electric immunity due to its part Ground-typing, which allows it to deal with Pokemon like Toxtricity better. Mega Steelix's bulk can let it live almost anything in a pinch barring using Sturdy.
Mega Sceptile
Mega Sceptile is B+ on the Viability Rankings. Mega Sceptile is used as a decent speedy attacker that outspeeds already fast Pokemon such as Crobat. However, there are some flaws it has, such as both STABs not really being great since Mega Sceptile gets walled by most of the Meta. Mega Sceptile can use Focus Blast or HP Fire for Steel-types, though using Leech Seed + Protect can annoy Steel-types as well as other bulky Pokemon.
TL;DR If you're looking for one best Mega for NatDex RU, it is Mega Houndoom due to its high speed along with being a really good wallbreaker. You can read this response on Smogon that ranks all of the available Megas in NatDex RU.
