I have preferred the frag grenade since Halo 2, and this hasn't changed in halo 4. "Frag and tag" was mentioned earlier and I'm assuming that means hit them with a frag and then shoot them in the face, which is what I love most about the frag. This takes skill and some great timing, but it means instant death for your opponent, and there are few things as satisfying, and with the explosives ability, it makes it easier to pull this off. This along with the ability to bounce them wherever you are creative and skilled enough to bounce them makes them the best choice in my book.
Sticky grenades are extremely tempting, but only for BTB or Dominion where vehicles play a vital role. My major complaint about stickies is the delay before it explodes. Sticking people is fun, but if you miss, you're dead.
I think that the initial radius of the pulse grenade does have the ability to kill a player, but I haven't tested it and can't until saturday. But Wheels and I did a little testing and found out that you can run through one and only loose like 1/8th of your shield.
UPDATE: A player can be killed by the initial blast and the final blast of the pulse grenade. The middle stage is purely a shield drain. Using these grenades to block pathways works because people are afraid of them, but in reality you can walk (not sprint) through one and loose less than 1/5th of your shield. So if you ever weaken some one's shields and they try to scare you with a pulse grenade, it might be good depending on the situation to truck right through it and kill them. They probably won't expect it, and you won't loose too much shield.