As I'm preparing for a once-in-a-lifetime UK trip, we are trying to find the best deals to prevent a once-in-a-lifetime bankruptcy filing. Living in Southeast Asia is great for saving money, but the UK is crazy expensive in comparison. But the wife wants to go, so that's what we will do before having children.
I found Ladybug.com because we decided renting a car will be most cost-effective and allow for freedom to see all the places we want to see. Their car rental prices pull from other sites too, but they were hands down the cheapest I've seen. It's not easy to find the cars that have automatic transmission only (I wish I knew how to drive manual, but I won't count on my grandpa's old truck lesson when I was 8 when I'm already going to have to drive on the wrong side of the road!). However, Ctrl+F allows you to search the page for "Auto transmission." If you can drive stick, you would save even more money than I did.
You can see how the comparison tabs give a good overview on what is available, although the filtering isn't the absolute best.
I also found that Ladybug.com linked me to compassflights.com, which I had never heard of before to get a steal of a flight back in the US from Miami to Atlanta. That price hardly showed up anywhere else. So right up there with Kayak comparisons, Google Flights, and SkyScanner, I'm putting Ladybug in my top travel comparison sites.
*Note: I haven't gone through with the car rental or flights yet, so I'm assuming it works. If the car rental extorts extra fees (what car rental place doesn't?) or flight doesn't get recognized (as my friend recently had happen through his JustFly.com booked tickets, twice), I'll let you know.