If you don't mindspending the money, the best kind is the metal can "twist and lock". SonicTronic, Sullivan and a few others sell this type, typically for $10 including a remote glow charger jack.

The next best one would be the rubber boot type that has a internal spring-loaded contact retainer. These work great, except the rubber do eventually get tired, and the 90deg bend doesn't allow used in some 4stroke engines.

Then there are the cotter pin type and other none-specific purpose contractions. If you want to go cheap, soldering a piece of wire onto a wheel collar works surprisingly well for 2stroke. 4 stroke might work also.