The best platform depends on the software you need to run and local politics :-)
Most new Notes software used to appear first on the OS/2 platform because that is what the Notes server was originally developed to run on. With Lotus support of additional platforms, more software is appearing simultaneously on other platforms with Windows NT being the second most popular in terms of software availability. However, as of Notes 4.x, Lotus has switched to developing on NT first because of the better development tools on that platform; some Notes add-ons only work on Win32 (Windows NT and Win95) and the Notes client works best on Win32.
Unix support is much better in R4, but the 3rd-party developers continue to write for NT as the main target platform.
For serious numbers of users per server, look at the AS/400 native port; there have been reports that over 10,000 users (using partitioned servers) can be placed on it without any problems.