However, a lab at the end of the hallway of the top floor of the rare book library in a musty old university that only a few antiquarians ever go to would be ideal. It is always a good idea to go for the ultra-modern or the antique. Remember that one can always fill the place with ancient manuscripts of futurist equipment as necessary. Ancestral estates, particularly castles, abbeys and mansions, are great, particularly if they are located on inaccessible mountains, in deadly swamps or anything likely to keep the curious at bay. Something mobile and subtle, a submarine or space station for instance, has obvious advantages.
Subterranean digs have the advantage of added security but can get a little damp and depressing.
Building a modern high tech facility has plenty of advantages, however the big downside is going to be cost. Overall, just remember that posterity is going to go for style as much as substance and something that looks good illuminated with lightning is really key.