So.. this is a first for me: I have two offers from great companies, one company already sent me a contract to sign and other will send it on Monday.

How do I tell a company that I will take another offer without being an asshole? (This are not big soulless companies that are full of corporate bullshit, so I don't want to be that guy that just leaves after getting a decent offer)

You pit them against them by politely asking to increase their bid. More money or vacation.

"Hey i'd love to work for you but i got two other offers that are monetarily much more lucrative for me. Would you be willing to match that other offer?"

@heyheni Not gonna lie, a bit more vacation time would be actually really nice for me. Thanks for the tip!

But in the end I would still need to reject one company and that's the part I'm not looking forward to :/

@Sebba you just say you got better overall offer. If they found better candidate than you, they wouldn’t think twice. You shall not also.

Exactly as @aviophile says.

Imagine: You're figuratively speaking on the towns market and you got 3 buyers at your market stall all wanting that same product. They start to argue who's get to buy your product. It get's nasty so you step in and say if you want this product are you willing to buy it at a higher price of x. The third leaves and searches an cheaper alternative. 1 and 2 agree to pay more. 2 even says he pays upfront in cash where as 1 says he doesn't have the money right now but he will double 2 offer. So you sell it to the highest bidder. No different for finding a job as a highly sought after job like a developer. 😊

Business is business. What do you think they would do if the shoe was on the other foot?

@aviophile Yea I guess you are right, thanks!

@heyheni Thanks, I'm just not used to denying job offers after last interview.

Dear Company/Representative

Thank you for considering me, however, another company I'm applying to have made a better offer. If you would consider <options that would honestly make you change your mind> to make your offer more appealing I would reconsider, otherwise I will have to accept theirs.

Best wishes,
Me.

Don't be afraid of referring to choosing the best offer as an obligation rather than a desire. It's a free market. Going with the most appealing option is the only universal rule.

@Nanos relationships are emotional. Not business. Treating it like business means you're hiring a prostitute.