Talking GGUF, Usually the higher you can afford to go wrt. quantization(e.g. Q5 is better than Q4, etc), the better. A Q6_K has minimal performance loss from the Q8, so in most cases if you can fit a Q6_K it's recommended to just use that. TheBloke's READMEs[0] usually have a good table summarizing each quantization level.
If you're RAM constrained, you'll also have to make trade-offs about the context length. e.g. you could have 8 GB RAM and a Q5 quant with shorter context, vs Q3 with longer, etc.