It is generally accepted that Big Money or some simple variation thereof (BM + Smithy, BM + 2 Library, BM + 2 Witch, etc.) is highly competitive when dealing with cards only in the base game.
Big Money is also very competitive against players who overbuy actions, and can even be competitive against strong experienced players trying to build complicated combinations if they do it in the wrong order. That is, it's easy to build a complex deck badly, but it's hard(er) to screw up playing Big Money.
Finally, Big Money tends to be more competitive in multiplayer, since in multiplayer there are more players competing for the same number of actions. So where you might get 4-6 of a hotly contested action in 2p, you might only get 2-4 in 4p. That makes it harder to chain together combinations. 4+p games also go much faster since there are fewer provinces per player; this rewards a strategy that ramps up quickly and gives less time and strategic room for comebacks, both of which are conducive to Big Money strategies.
That being said, a strategy that buys nothing but money and VP is no-doubt a long-term loser on the vast majority of 2p boards.