I put strider down since I don't remember having trouble with any of the enemies from HL, especially nihilanth, he was a piece of piss. Yes the Gargs were damn tough, but they were damn stupid too.
And ok, yes the ichthysaur was tough and scary, but it did look totally ridiculous. I honestly thought it was a sprite(ie not a 3d model) when I first saw it!
I had to give my vote to the strider, I know they were easy to kill (easier than the gunship), but they were damn cool and last time I took them on, on hard, they were a real pain- I was constantly faced with a barrage of flying automobiles and stuff.
But yes they were sort of ruined by the stockpiles of rockets everytime you got to one, I think they would have been scarier and more fun to kill if more of your weapons could deal them damage. Obviously the rockets would do the most damage, but if you could chip away at them with more of your guns and didn't have the luxury of unlimited rockets ( just a couple of reloads worth)- I think battling a strider would have been much more enjoyable.
But I still voted strider because I saw them do so many cool things, they can definitely be an intimidating opponent and they make a bloody satisfying noise when they die
A strider should be able to shoot down rockets.. Then it's the most dangerous enemy by far.. And the Garg was easy, tentacle 1 grenade and it hides.. I don't know...