It's not easy to choose a good editor. First you have look which device you use (with IC6581 or IC8580). Most people swear by 6581 SID, but my opinion is that the IC8580 sounds much better, cause i can better play around with filter settings. (IMHO the 6581 sounds best without filter). So decide by yourself ;)
I heard that the best editor for the C64 Stuff is GT2, but it's for PC's and i don't use it. I want to listen to the original device by composing ;) so i use DMC7. 10 years old but nice software with ok rastertimes and cool filter sets.
Problem is that, if i want make music for games or something else on C64 device - i've look out that the tune sounds good for both ICs ;) e.g. The "Robotic Walk Engine" tune sounds crap for IC 6581, so i've to correct or remove most filtersets :(
I don't know if another editor can handle these things better...