I would say that depends totally upon your usage.
Li-ion Paslode nail GUN nailer batteries are probably the most hassle free. They are light, deliver high and constant power and can be charged at will. They loose charge very slowly, they charge fast and can be used partial charges. They deliver the maximum no. of charge cycles, i.e. the longest battery life.
NiCd batteries save the purpose adequately if you are using the tool frequently. They get discharged on long storage so you need to charge them for several hours before you can use your too. If this is not a problem, NiCd can be a good choice as well. They are heavier than Li-ion, loose power during use. But on the upside, they are much cheaper than Li-ion Metabo cordless drill batteries and have a decent life.
NiMh batteries are not very popular in tools since they are more expensive than NiCd and do not offer those many additional benefits. They are lighter than NiCd but have a shorted life span.
Technology is constantly improving and batteries are getting better. Newer NiMh batteries have a lower discharged rate, come with fast chargers and are cost effective. A lot of work is being done in the area of Li-ion batteries and they are getting more advanced.
Just remember your work needs and match them to the right tool. For example, Li-ions work well in the cold as well, NiMh does not. Similarly, NiMh can take higher temperatures, Li-ion cannot without suffering in some Milwaukee power tool batteries.