Board games

Kodaman (build a tree!)
Machi Koro (build a city)
Camel Up (betting on camels in a camel-race)
King of Tokyo (let your monster fight against other monsters)
Evolution (evolve your animal to get the most food)
Dixit (association game, really beautiful images!)
Istanbul (trading game)

I find Gravwell to be really fun, can only be played with a max of 4 people, but the games are fun and swingy and requires a surprising amount of thinking to play efficiently.

I've also been really in to Race for the Galaxy ... it's a bit complex to explain, but is a lot easier to understand than it first appears once you've played it a couple times.

On Steam:

Smash Up (card game/smash 2 factions into 1 deck)
Small World (My favorite game)
Ticket to Ride
Gremlins
Hive (abstract,strategy)

Real Life?:
Coup (cutthroat)
Resistance Avalon (minimum of 5 players, cutthroat)
King of Tokyo (from the creator of Magic the Gathering/dice game)
Splendor
Epic Card Game (from the creators of Star Realm)

I advice you to watch the following youtube channels for more board game stuffs:
GeekandSundry's Tabletop
Tableflip (Gamegrumps crew)
Watch it played
Thedicetower (ehhhh but good enough)
rahdo

real life board games are the best !
(also i have talisman (good) and mysterium (meh) on steam)

Ever try Heroscape? Shame it is no longer being made. =(

Same with Dice Masters... =(

Have a look at Shadows over Camelot. It's a cooperative board game where basically you and your friends play as the Knights of the Round Table. Basically you rush from quest to quest, plugging holes in the quickly sinking ship that is Camelot.
Also if you are looking into some card games, check out Citadels!

A fairly casual game would also be Settlers of Catan (without the expansions is more than enough).

Steam game hmmm, Jackbox Party Pack maybe? Don't know too much for Steam. Give a shot at Town of Salem, it's basically online mafia. There's a free version on their website, steam just monetizes the skins.

This thread is showing a distressing lack of Hero Quest! No, really, Hero Quest is simply the best game ever made. You can't argue with any of that now, can you??

Joking aside, I can't really help with your particular needs outlined above but I'll drop in a recommendation for a properly gorgeous and great tabletop 4X in the form of Scythe. It's not a deckbuilding game nor would I call it casual, but it is a great lot of fun and the lovely aesthetics and thematic backdrop are an added plus (if you're into them, of course).

Seconding Thunderstone for deckbuilding and Betrayal at the House on the Hill (horrible name, fun game) for Arkham-lite. I wish I could suggest more things since most of my time when seeing friends is spent playing boardgames but my mind is super cloudy at the moment :/

About Steam boardgames, 100% Orange Juice is super popular among weebs but I never played it so I can't say if it's actually good. Digidiced and Asmodee published some boardgame ports on Steam. Mysterium is OK, although the AI is terrible. Never played Talisman on Steam (you'd have to be either coaxing or threatening me to get me to play this glorified Goose Game) but it's supposed to be faithful to the original. Also you might want to check Throne of Lies if you like hidden role games like Werewolves, Shadow Hunters or Bang.

Reminded me of this puzzle:

http://www.itstoohard.com/puzzle/f6mQq9QH

Thanks for the replies everyone, I ended up buying Pandemic but I haven't had chance to reply to everyone yet! Thanks for all the suggestions though!

It depends on the style of the BG you like most. For example, I'm a family and strategy guy, but not heavy gamer.