There is no single best carpet cleaner solution for stains. There's a best one for red wine, a different one for bacon grease, and a very different one for the spot the dog left behind the couch. Anybody selling you one bottle for everything is selling you a bottle.
After 30 years of pulling stains out of carpet around Cypress, this is how we'd sort it.
Start with what spilled
Stains fall into a few families, and each family needs a different approach.
Water-based spills (juice, soda, coffee, wine) respond to blotting and a mild, low-suds cleaner. Get to them fast and most come out.
Oily spills (grease, salad dressing, makeup, crayon) laugh at water. They need something that breaks oil down, and they need it before heat sets them.
Protein and pet messes (urine, vomit, blood, milk) are organic. Detergent leaves the organic material behind, which is why the smell comes back on a humid August afternoon. These need an enzyme product.
Two rules cut across every family. Blot, never rub. Rubbing frays the fiber tips and spreads the spot. And test any product on a hidden patch first, inside a closet say, because some cleaners pull dye out of carpet just as happily as they pull out stains.
Homemade solutions that earn their keep
You can handle a lot with what's under the kitchen sink.
Fresh water-based spots: clear dish soap, just enough to tint a cup of warm water, dabbed on with a white cloth, then blotted with plain water to rinse. Skip colored cloths and colored soap. Both can transfer.
Light spots and general dinginess: equal parts white vinegar and water in a spray bottle. Mist, wait a few minutes, blot. Mild enough for most synthetic carpets.
Grease: sprinkle baking soda or cornstarch on the spot, let it sit 15 minutes to pull the oil up, vacuum, then hit whatever remains with the dish soap mix.
Set-in stains on lighter carpet: a cup of hydrogen peroxide with two tablespoons of dish soap can lift them, but peroxide is a mild bleach. Test first, and keep it off wool and dark carpet.
The one thing every homemade recipe has in common: rinse it out. Soap left in carpet dries stiff and tacky, and tacky carpet is a dirt magnet. Run your palm across a spot somebody cleaned last month and never rinsed and you'll feel it, a slightly crunchy patch that goes gray again every few weeks no matter how often you vacuum.
The rental machine problem
Grocery-store rental machines are fine for a surface refresh. For stains they're a poor choice, for two reasons.
First, they put down a lot of water and recover only part of it. In Gulf Coast humidity, a carpet that stays damp for a day or two is a mildew smell waiting to happen, and any pet stain underneath wicks right back up to the surface as it dries.
Second, the detergent they're sold with is heavy. Most people over-dose it, and the machine can't rinse it out. You get a clean-looking carpet that resoils in a month.
Renting one anyway? Use less detergent than the bottle says, go over each area with plain water afterward, and run fans until it's dry.
When to stop and call somebody
Some stains are past the DIY line. Pet urine that's been there more than a day has soaked into the pad, and nothing you pour on top reaches it. For those we use a live enzyme pet treatment that goes after the pad itself, which is where the source is.
Old stains that have been "cleaned" several times usually have a residue problem sitting on top of the original stain. Anything bigger than a dinner plate, anything on wool or a natural-fiber rug, and anything you can't identify is also worth a phone call before you make it worse. Bleach spots and dye damage are the exception; no cleaner puts color back, and we'll say that up front when you call.
Our regular carpet cleaning is low-moisture and soap-free. Carbonation does the lifting, so no residue is left to attract dirt, and the pile needs about an hour to dry. Three rooms are $88 at the moment; the coupons page has the fine print. If the spot has a smell or a history, call 281-746-6055 before the next round of scrubbing. Until then, put a dry white towel over it and a book on the towel.