Best Road bike for 2,5k

You could go with a Trek Madone 3.1 for that price (~ $2,039) or you could increase your budget a bit and get a Cervelo S2 (~ $2,800). I highly recommend getting the Cervelo. The Felt F5 is also a decent bike (~ $2,299), but I would get the Cervelo over pretty much anything else.

http://cervelo.com/en_us/bikes/2012/S2/

A Cannondale CAAD10 5 (Shimano 105) for $1500 and $700 in your pocket to spend on whatever else (power tap, wheels, beer, etc).

I got the CAAD10 4 (Rival) for $1400 through a club discount and could not be happier.  My bike is as light (17.2 pounds 50cm) and stiffer (IMO) than many of the carbon bikes in the $2500 range including the Felt F5 mentioned above.

I just bought a Felt F4 in March. I considered the CAAD 10, which also a great bike for the money, but I opted for the Felt and I really love it! 

Ultimately, however, what is best for one person may not be best for another.  Try some bikes out and see what you like.

Witness!