Reply explaining why

Fire! The visual is more rewarding. I like to see them catch on fire and hear them grunt as they try to escape from the flames.
Yes, I'm sick.

Frags overall, I get the most kills with them.  Next would be incendiary, especally on red.

Frag. Instant kills.

That really depends on the map. On public servers I usually equip the incin more often than the frag..

My personal favorite is hearing the loud bang of the frag grenade followed by the warm chime of the kill sound.

I voted for frag, although it's a close match between frag and incendiary. The incendiary grenade is useful because you can stop rushes with it, and for defense (keeping them off your back as you run with the briefcase ) However sometimes it's not really useful for killing people all the time, as people can often get away in time. So, in elimination I usually go for frag, and in CTF and Inflitration I go for incendiary. Then of course the smoke and flash have their uses..... Argh, too many choices!

The most fun experiences with a grenade for me have been:
Smoking up the whole lecture hall area on Shop, and running inside knifing 2 people, and having one of the enemies shoot his own teammate
4on4 elimination on Raven. I lobbed a frag straight down a hole in the roof and managed to get all 4 of them, winning the round instantly  
me and a friend were playing against a bunch of bots (2vs16). They got stuck in their base all together, and u can imagine what happened, needless to say I got 16 kills in about 3 seconds with an incendiary. roflmfao  

frag... i wanna see the pieces from them

I the fire starter, i'm the instigater......yea