Model Ms feel the best to me but are too noisy for most work environments. Topre RealForce (I prefer the 55g version) keyboards are more expensive but feel almost as good (albeit different) and are quiet enough for any office. I use a Model M at home and a Topre RealForce 55g at work and am satisfied with that setup.

Both are extremely well built and will be long-lived. I've been using my Model M since 1984.

Neither is ergonomic, if that matters to you.

Both come in full-size and tenkey-less versions.