#1 Acquire
After that it gets muddled. I *really* enjoy Monopoly when people understand deal making. There are so many more deals than simple trading of properties + money. If people ever say Monopoly takes forever to play then they are playing it WRONG.
Beyond Monopoly, I am starting to dislike games where your entire premise is maximizing turns (Agricola for example). Yes there are many strategies in playing these types of games but it all comes down to maximizing your production/farming/profits/whatever through a given set number of turns.
Dominion, while a card game has a flexible ending point. You are trying to optimize your deck but in one game you might have a lot of time to do it, others you will be rushing to even buy a Duchy to win it... I have played a game where the high score was 2!
The "Euro" games are really beginning to saturate the market. While I love the fact that boardgames are going through a revival, it is really a hard sell to buy another railroad game that plays similarly to other railroad games... If I am going to play a railroad game then I have my 3 picked out... light days I will do Ticket to Ride, medium days I will play Rails of New England (sloppy rules but it is a really fun game when you figure them out), and if I want epic then I will go with Railroad Tycoon. The saturation makes it hard for another game to fit in but Rails of New England is a new addition to my favorite rail games as it is a completely different mechanic than I am used to as a Rail game (sort of plays like Power Grid).
Anyway, sorry for the longish reply. If ANYONE is up for a game of Acquire, I am always available!