I got cards from both Megafon and MTS and have been using them for 4-5 days, first in the northern outskirts of Moscow, and then in the Vladimir, Yaroslavl, and Vologda regions. MORE>>
Generally recommended: Shell and Lukoil.
Lukoil has been recently involved in scandals with one of its top dogs causing a fatal car accident and attempting to cover it up or something of that sort but the question was where to buy gas/diesel, not the luxury of making a statement.
Generally NOT recommended as of the last 2-3 years MORE>>