Oh... I meant ingredients wise... in other words, home made boosters for the pre-made cans of Mr. Beer LME et al.
Honestly, that's like asking what the best booster for premade canned spaghetti sauce is. Sure, you can add some fresh herbs, but it's still canned spaghetti sauce. Not much you can about it, really.
Haters gonna hate on the Mr. Beer.
I'm no hater, because I think it's a decent way to decide if making beer is for you. And a few people love the results. My best friend loves her Mr. Beer beer. While I rarely turn it down, it's just, well, quite frankly not that good. It's not terrible, but it's just not that good. I'd rather have just about any commercial beer rather than her Mr. Beer stuff.
Going back to the spaghetti sauce analogy, think of it this way.
Mr Beer= canned Hunt's spaghetti sauce (not too terrible)
Extract kit with steeping grains= jarred Prego sauce (better)
Partial mash kit with extract= "homemade" sauce with tomato sauce, seasonings, fresh ingredients, tomato paste (can be excellent)
All grain= homemade sauce with fresh tomatoes (can be good to excellent, but more can go wrong!)
It really depends on the person. Some people are satisfied with the Mr. Beer or Cooper's kits, but most think, "It's ok" at best. Adding sugar/booster/hops to something that is just "ok" at best isn't really going to take it to the next level.
If someone makes their Mr. Beer kit and loves it, like my friend, I would never criticize it. But it was never for me. I started with a Beer Machine 2000, and still have packages of the mixes downstairs. It never made something I wanted to drink, but I know many others enjoyed it so I would not disparage it.
I just want to give my opinion on the quality you can expect from certain products.