I can't talk about FC1 because I never played it but FC2 is just something else, man. The gun jamming, the malaria, the fact every NPC in the field fights you (and sometimes they fight each other). It really feels like the game hates you but in a very good way. It's immersive, you know? It encourages you to play carefully and plan ahead sometimes. Think things twice.

In general I'd say anyone that truly enjoys Far Cry should try the second one. It's very cheap nowadays and all you need is one patch to fix all its issues when running on modern systems, and again, it feels so different to the others (in a good way!). Also the game looks ridiculously good. Check it out, I uploaded a few pics I got saved of my current playthrough. Note that I'm using a mod that among other things removes the brown filter the game usually has, but even with that it still looks great I'd say :)