I like Challenge tires because of their KOF look. They are hand made in Italy and are the successor of Clement, if I am not mistaken. I used a pair of their Parigi-Roubaix clinchers at L'Eroica this year without any problems. Here's a link to their tubular line:
http://www.challengetech.it/
Veloflex also makes a high quality 700X24c tubular tire called the Roubaix. Here's their link:
Good luck!
Michael Haddad
(Snowy) Brooklyn, New York, USA
Michael's Professional Bike Repairs
http://www.mikebikerepair.com