Serious Sam [GOG] and Serious Sam Classic are the same games. Classic game with the original physics in some levels.
HD version obviously are remastered versions for the modern PC with normal widescreen suport, good models and textures plus some new secrets. It losted some physics puzzles from the classic versions.
Revolution is the compilation of classic games + some new secrets + widescreen + ressurected online + new campaign. In theory.
Fusion is the compilation of HD versions and SS3: BFE + engine improvements that you'll not see in "original" HD versions.
So, what are the best versions for playing? It depends on your desired expirience.
If you want classic gameplay, interface and physics - use CLASSIC (GOG or Steam). But there is no good widescreen support, all text will be too small for reading (1024x768 is the best choice, I think).
If you have x86 PC, but want to play with widescreen and easy to use multiplayer - you should use HD VERSIONS. There are some minor fixes that will do game easier, but it is not critical.
If you have x64 PC and want widescreen, you can play FUSION. You'll also receive new graphical effects.
Don't use Revolution, it is peace of... Bad...
New campaign is still not available (5 years). It is still beta access. There are no updates for 3 years. Game still not have normal widescreen support with scaling text, it is not possible to read text on 1080p. Funny, but this game added new bugs (hard to jump from water, for example).
Post edited April 29, 2019 by StalinEXE