Half-Life or Half-Life: Source?

Play the original Half Life first and then the source remake.

And then you can play Black Mesa to top it off.

source actually its the same game but source got better graphics

play the black mesa mod, then play the last levels in source

There is an (official?) updated models pack available for HL, Opposing Force, and Blue Shift that isn't available for HL:Source. From personal experience, characters look a lot better in Opposing Force and Blue Shift than they do in HL:Source.

Half Life 2.

You can play Black Mesa too. It should be in steam already, I wonder if they are waiting for the last level or something.

if youre a big valve fan then buy both or if you like HD textures then hl source or if youre old school player play the original

Original Half-Life. HL:S feels like it has to load new map every few steps. You cannot bhop thru the whole chapter like you can in the original HL.

or just go with http://www.blackmesasource.com/

http://steamcommunity.com/sharedfiles/filedetails/?id=94128543

its fine... and than you get to the jumping puzzles and it all comes back to you...
dont think i would reccomend playing old Half-Life to anybody with thin nerves...
it was great in 1998... not so much now...

Half-Life. HL:S isn't that much of an upgrade and, at least for me, playing an older game is a better experience. If you can't stand those graphics get yourself a HD pack, which is available on Steam.

Half Life: Source looks quite a bit better, but the jump in graphics quality is mostly negated by the Half-Life High Def pack (Hope that link works).
I played both for a little bit, up til the box-on-chains jumping part. In HL:Source, I felt like everything was covered in a thin coating ice, everything felt too smooth, more like you were sliding around than walking. Normally that wasn't too big of an issue, but I fell off the boxes (and slid off the giant elevator) FAR more times than the original game.

Since the big draw of HL:Source was better graphics, and HL:HD Pack brings the original closer inline, I'd say go for the original. Plus, there are more mods for HL than HL:Source.

Go with the original.

Half-Life WON retail.
Dunno but original HL just not been the same for me without Sierra + Valve intro and that classic menu

Black Mesa-

both