If you want to be good, then you don't pick your equipment based on what you like. You pick it based on the situation.
Claymores to guard your back if you are sniping or playing defense with a LMG. Betties if you are doing this outside bcuz the grass/terrain often hides them better than claymores and they have a 360 degree trigger.
For all other mid-range classes I use frags, and for rushing classes use semtex. C4 has a large blast radius, but half the time it doesn't go where I want and I end up getting killed while trying to retrieve or detonate. Sometimes I kill myself and several enemies/team-mates at once with it. So I tend to stick with the frags/semtex.
Also, even if you favor set-and-forget explosives, if you find that enemies are using engineer/emp/blackhat or some combination thereof, it is good to have a class available with nades/semtex. I can always tell when I am killing the people who have all their classes setup the same with only different guns.