Speaking for myself, both extremes are tough for me. Like I said, triplet is more accustomed to writing these things on a weekly basis than I am so she may feel completely different about it. I tend to be my own worst enemy when it comes to writing though... I'm a little OCD about certain things.
Progeny was tough because I didn't love it and had a hard time finding things I found interesting enough to talk about. I don't like reviews that are basically glorified recaps, I want to learn more about an episode when I read someone's unique perspective. So, Progeny was hard to start, and ended up being the shortest review I've written. Oddly enough, Subterranean is another ep I didn't love so much, but I found a lot more to talk about in that one - probably because it was SO much more Clark-focused.
Promise and Nemesis are both eps I adored, but with them there was almost *too* much I wanted to say. So, tough again.
I guess I find them all a little tough, but I still enjoy the process.