I think GOW2 is the best due to its fantastic level design. The game was also fun pretty much the whole time, whereas the first game had some issues for me near the end - Hell and that level with the spinning blades in the ceiling... ugh. I still love the first game though and the third game as well, though it's my least favorite of the three. It definitely has some epic moments - the whole Helios level is just awesome and many of the boss fights (like Hercules) are amazing.
I definitely like the original trilogy better than what they did with GOW 2018; not that it's a bad game, it's just not GOW to me anymore. I gotta admit though... I'm still anxious to play Ragnarok, as it does look incredible.