HL laid the groundwork for all modern-era gaming, including Deus Ex.
The original Deus Ex was nothing like HL. And HL never inspired or became the groundwork for Deus Ex. While DXHR is a 'modern shooter' it is not a proper DX game compared to the original. It only managed to be better than DXIW. So I'm not considering HR to be a real DX game. It is like IW. Some people consider IW to be a good game in its own right. So is HR. They are both 'just not deus ex'.
And, while HL is fun to play the first time, it has little or no replaybility. After 10 years, I can STILL find secrets in Deus Ex that I never saw before. Now that is a feat which neither Half-Lives nor Human Revolutions can beat.
IMO, DX1 (the original) > HL + HL2 + HL2E + DXHR + IW