I honestly think Demolition if your good enough to get tons of kills / defuse / plants. You can join a game with people you want to play with to insure you win even though 90% of the players you play against aren't good, or are actually beginners.
Deathmatch is good but you need over 50 kills to get enough points to quickly rank. If a good Bonus Gun comes up use it, or if you're really low on health go for it. Also turn off Random Weapons.
As said above Arms Race is probably the fastest, but also has the greatest chance for you to fail miserably and only get 50% of what you could in a Demo / Deathmatch.
Casuals ... Avoid at all costs... lol yeah but if you dare they offer a decent amount too, just not worth the mic spam, trolling, mission-mongers playing them...