I find the most intense part of a trip is if you find yourself in super-fast-forward mode. This is something that happens most often it seems if you burn the spice. Most people who have experienced it will know exactly what I'm talking about, and it's kinda self explanatory. Everything is moving incredibly fast and you're usually at the mercy of simply waiting it out. The intensity is in the air, and the crazy part is that the fast-forward effect doesn't seem to be restricted to the visions as one might expect.
I remember one night when it was nice out, I went out on my terrace/porch area (second floor - out of public view) to have a quick journey. As I was taking my hits, I was unexpectedly joined by two friends who just wanted fresh air. I closed my eyes and wound up with that fast-forward mode in action. My friends began having a conversation, and I noticed that their words were flying by a million miles a minute. I opened my eyes and surely enough they were moving around in jerky fast-forward as well. I had always assumed that the visions were sped up, but that experience made me realize that it was my perception of time itself that was thrown into full-throttle. This is fascinating to me because on other psychedelics, I have commonly experienced time warping (most often time dilation - time slowing down) but that warping doesn't usually apply to the scope of immediate experience. So in other words, it's more often like I look at the clock, it's 10:00; feels like an hour goes by; was really just 15 minutes. Even if time speeds up, it's still on that larger scope where perhaps I'll think 10 minutes have gone by, but it's really an hour.
Now, that is the effect that I've most commonly experienced and find to be the most intense, but the other side of the same coin is equally, if not more (and most definitely more) intense, and that is the experience of eternity. I have only received a beloved taste of eternity but once, however it was incredibly profound and intense. I was soaring through white light, and as I approached the godhead, time began rapidly slowing down. I could literally feel/sense the particles of time growing bigger until I got so close that time had literally stopped. Of all the experiences, that is probably the most ineffable even though it contains such simple and common mystic elements. So I guess when it comes down to it, between super-fast-forward and eternity, I find extreme time warping to be the most intense characteristic of a trip.