getting kills with throwing knifes, is way more satisfying then C4, if you can be good at it, then it looks pimp

They're both great (c-4 and TK), but there is nothing better then nailing someone with a TK, picking it up, and nailing someone else with it.  Plus, I play lots of SnD so the TK is even better for it's stealthiness (new word, you can borrow!)

All of the lethals take a certain amount of skill.

C4 definitely takes a lot of skill, and is probably the most rewarding. You can plant it discretely and detonate from afar, or toss it over obstacles to get pesky hiders. Jump throwing C4 is definitely a skill. You need a lot of coordination to execute and get the right flight.

I would say claymore takes a lot of skill. It's hard to place claymores such that the laser cannot be seen and such that if the person is running by that they'll actually be killed by it. When used correctly it can be great though.

Frags and semtex are a little different. Frags can randomly bounce and give you lucky kills. I would say cooking a frag is skillful though.

TK definitely though. Obviously you need a lot of skill.

I would say betty takes the least skill. You just plop it down. Obviously you can put it in unobvious places, but that's just about it.