It really does depend on what you want as far as complexity and play style go. If you and your group are willing to tackle aggressive and brutally efficient systems, my first suggestion would be Race For The Galaxy first and Core Worlds next. Your first game of RftG is likely to take an hour, but pretty much immediately after that, assuming everyone groks its mechanics, games tend to go for 30 minutes flat. Core Worlds will take a minimum of 60 minutes for 2 players, 90+ minutes for 3 or 4, for the first game, 120+ minutes for 4 or 5 so I'd stay away from 5p until there's some mastery among players. Legacy: The Testament Of Duke de Crecy is also a heavier card game driven by worker placement that is all about building a family tree, 60 - 90 minutes, 4p, that I rather like.
Johari, a game of gem trading and noble hiring is almost pushing at the difficult/complex end of the scale but isn't quite as thinky as those listed above, but still, it hits far heavier than it looks, 4p, 60 minutes. Also pushing at this level is recently reissued Traders Of Carthage/Traders Of Osaka which features cards with multiple uses, also about trading and successfully selling goods for points - 4p, 60 minutes.
Medium games would probably include things like Design Town/Flip City, recently published by TMG (as Flip City). It's quite a clever reductionist deck builder that's pretty cool to play - different enough to Dominion and others of its ilk, plays up to 4p, 45 minutes. Mundus Novus is an interesting 6p game of trading and lite engine building that plays in about 45 minutes. Condottiere and Artificium, both play up to 6p and are decently interactive with interesting things happening in them.
Hitting a little lighter are things like PAX by designer Bernd Eisenstein, 4p, 45 minutes that plays quite clever for its heft - tableau and engine building. Port Royal is a really great push your luck/lite engine building game that plays 5p in about 50 minutes, and Dragons Hoard is essentially a massively cut-down Race For The Galaxy and a whole lot easier, plays 5p in about 45 minutes. I'm also partial to the very clever We Will Wok You full of delightfully eyeroll inducing puns about buying ingredients and woks to cook for a Wok Show and feed Wok Stars... (crickets) 4p, 30 - 40 minutes and great fun. Also I have to mention Australian designer Phil Walker-Harding's Sushi Go which is a more fun and thematic card drafting game than 7 Wonders (that I'm thoroughly sick of), 5p, 30 minutes.
At the easier/filler end of the spectrum, I'm a big fan of Coloretto, R-Eco (R-Oko for the German edition that has better art) and Parade, also recently published by iello, Knizia's Friday The 13th - all of these revolve around trying to not take cards as much as taking them, and they each mechanise a little differently, all worth a shot.
There are tons more, hopefully that's enough to get you started. I have a ton more I've recently acquired but haven'nt played yet so I'll report back as I do.