Which solder for ESC?

[Npdp72]

Does anyone have any knowledge of the best type of solder to use for wiring up ESCs?

The solder I have been using recently is lead free, but it doesn't seem to be as easy to use as solder I have used in the past, which I'm guessing was either 60/40 or 63/37.
And I have had some trouble with joints getting hot when in use in the car.

I've done some reading up on 60/40, 63/37 and lead free solder but would like some specific advise relating to this application.

Which is easiest to use?
Which gives the best (most conductive) joints?

Thanks.