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I would say Jolteon and Flareon would be the most dangerous to look after, but they also seem like some of the most useful.
Jolteon could be used to charge and power electronics. Power outage? No worries, Jolteon can probably do something about it.
Flareon can heat food, and provide light in the form of candles, and is the ultimate snuggle buddy.
The easiest would be Leafeon, provided you live somewhere sunny.