Anyone know of any good authentic sounding horns and/or horn sections that sound convincing?  Everything I've tried so far including the horns on my XV-88 and all my softsynths sound....well....fake.

These are the only ones you'll ever need! :)
http://www.samplemodeling.com/en/products.php

I've got them and it's AMAZING what you can do with them! :)

Check out these video demonstrations of the trumpet and the trombone, for example:
http://www.samplemodeling...n/products_trumpet.php
http://www.samplemodeling.../products_trombone.php
 
 
EDIT:
sorry.. i just linked to the trombone and trumpet when you asked for horns...
 
here's their tuba and horn library:
http://www.samplemodeling.com/en/products_horntuba.php
 
and here are some demos:
http://www.samplemodeling.com/en/demos_horntuba.php

Question: When you do your horn parts, do you do them all in one pass?  If you haven't tried this already, play each part one at a time while mimicking the phrasing of a horn player.  Don't quantize the tracks.  The mistakes are what make it sound real.  
 
My $.02.

Thanks for the replies.  I appreciate it...!

Chris Hein Horns Pro - http://www.digifreq.com/digifreq/newsinfo.asp?NewsID=4209
 
Scott

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If you like hardware:
 Yamaha Ex5r (has a VL1m engine, monophonic) or VL series (one or two voices)
 Korg Karma/Triton with a MOSS expansion (6 voices), Z1 (12 or 18 voices) or Prophecy (monophonic)
 
Basically anything that uses Physical Modeling will produce *VERY* realistic brass and horns.
I use a Karma+MOSS (6 voices) and myEx5r's VL engine for horns. Its very good. :D
 
There is also the Technics WSA-1, but that one uses samples as the starting point and is dubbed "Acoustic Modeling" which isn't the same, nor as good.

Session Horns for Kontakt 5 ( I believe it works with the free Kontakt player as well)

Making them sound real depends a lot on how you "play them" when you record them. Its more than having a good sample, although that helps. If you're just entering notes on a the PRV or Staff view, you will have to do a lot of work to make them sound real. Making horns (or any instrument) sound real involves giving them articulation, dynamics, phrasing, etc. That's what Frank Zappa called "putting the eyebrows on the notes." Its much easier to get real sounding horns using a synth keyboard (unless you can't play a keyboard). 

yamaha's VL70-m with the patchman chip upgrade along with a wind controller is about as good as it gets.

I can't speak in comparative terms to the many suggestions here. I would just add that I have the Garritan Jazz soft synth and the horns sound really nice.