Portal (and its sequel) are the only Valve games I've enjoyed. I've tried to play HL2 a couple of times and just can't stay interested much past Ravenholm. Shooters just aren't my genre of preference.

I think the use of Arabic numerals for the Witcher series and Roman numerals for the Two Worlds series is adequate for differentiation. And it matches the respective box art, so there's even a justification!