I bought one about 18 months ago, didn't do a lot of research just asked my electrical supplier for one.
Expensive but I got the Hulk HU6511, sometimes my cars sit for a while and need a jump start. It has started my LS1 about 4 times, 2.5L Subaru 3 times and Harley 3 times without a recharge.
Originally I bought the smaller model and it wasn't strong enough and the guy I bought it from allowed me to swap it for the bigger size.
Prices vary a lot, I think get the biggest you can for your budget.
Better solution is drive the car more and don't need a jump start but I'm not getting that right