I'd be a bit careful with buying someone a board game if they are avid players. And I would never buy them any version of Monoploy because in that community Monopoly is everything that is wrong with board games.
If they love playing board games, there is a possibility they have hundereds of them.
And game dependes on what kind of games they love - easy, medium, complex, war, euro...
If you start talking to them, they will probably tell you about games they like and the games they have on their list (every gamer has at least 10 on their wish list). Without that, it would be impossible to guess.
I like easy and medium, mostly family board games and on my wish list are Concordia, Hansa Teutonica, Alchemists.
This is the list of most popular board games. As you see, you can lose yourself in it.
boardgamegeek.com/browse/boardgame
Tell them you would like to join them on a board game night. There is not one gamer who will say no.