I would argue there is no background that clearly stands above all else.
And what do we mean with best base? Is it a matter of technical background or best environment for forming the mental makeup of a future aspiring mma fighter? And clearly these two factors depend on where you live. Early teenage judo in Russia, France and Japan might be much tougher and high level than in Canada. Boxing in Cuba and America could very well be more technical than if you start boxing in Africa.
Some sports might be very suitable as a background but the sport is not widely popular and therefor the competitive pressure and also the level of coaching is lower. The best background is also individual, Khabib's genetics clearly make him a grappler. Sambo, wrestling and judo are much better background for him than boxing. And talking about Khabib, he is also an indication that cross-training from a rather early age might be the best,
If we are to focus only on the technical aspects of an average person I suggest this ranking of backgrounds:
1. Sanda.
The distance and stance is reminding of MMA. You fight generally on the outside but depending on your game plan and reach you might want to enter the boxing range or go for a takedown. No long grappling exchanges are allowed but you will see throws, trips and single legs. A child or young teenager will learn a lot of body coordination that will translate well into MMA.
2. Combat Sambo/Sambo
A next to perfect storm of takedowns, quick submissions and punches. The grappling is somewhat GI-dependent but the trips, double legs and single legs make up for it. It is my belief it is better to focus on takedowns and achieving a dominant position than learning all the complexities of BJJ early on.
3. Greco-roman
MMA is fought standing up and not bent over. With greco you are likely to dominate the clinch and the wallgame.
4. Kickboxing. Strong fundamentals will get you far. Sure there are no takedowns and some muay thai moves are not allowed but the focus on hands and a stance more similiar mma will make up for it. The aspiring mma fighter should not start grappling too late. There are exceptions like Izzy or Alistair.
5. No GI-jiujitsu. Despite berimbolo and butt scooting it contains tons of mma fundamentals which you need to learn in one stage or another.
I think the key to success is to start in one sport at a young age(10-13, or younger) and then slowly starting training a complementary art some years later. Then you want to start having MMA amateur fights at 18 if not earlier if your country/state allows those competitions. In american setting this could mean that you do folkstyle but in your year junior you to start exploring kickboxing and start having some fights in your last year. In Russia you might start out in kids sambo but in your mid teens you introduce boxing or kickboxing and learn the fundamentals+get some competitive experience.