There are many choices and much depends on your computer platform (PC/Mac, interface connection (internal card, Firewire, Thunderbolt, MADI, etc.) , software preferences, and budget. Do you need a system with built-in mic pres or just line levels? ADAT? What kind of analog I/O connections, etc., etc?
A 24 channel, high-resolution converter system can be a significant investment and should be tailored to your specific needs. Everyone has his/her own personal brand preferences so you need to do some research to find what fits your needs and budget.
Some to look at (covering a wide range of budgets):
MOTU 24 I/O Core
Echo Audiofire 12 (2 required)
Apogee Symphony (Expanded to 24X24)
Focusrite Saffire 56 (using ADAT I/O)
Pro Tools l HD
Pro Tools l HDX
RME M-32 AD and M-32 DA
Lynx Aurora (expanded)