It depends on what you say about the business. If you accuse the business of professional incompetence (for example, "the vet murdered my dog by stabbing it in the heart"), you can be sued – this is in the realm of per se libel, where they don't have to prove that they were harmed. On the other hand, you can defend yourself in the suit if you can show that the claim is true. (You claim that the statement is true, plaintiff presents evidence that it is false, the jury decides whose evidence is more persuasive). If you prefix the statement by the expression "In my opinion,...", it's the same, since merely saying that it's your opinion does not convert a defamatory statement into a non-defamatory one. The standard is not that you have to actually assert a false damaging statement, you can be liable for defamation if you imply a false statement. Any statement which would be understood to be a statement of fact and which causes damage to another person could be the basis for a defamation suit, such as the statement "Dr. X supports Proposition X" (where that is a financial death-sentence in the local political context), and again, if the accusation is true, it is not defamatory.
Some negative statements especially web page reviews spew negative attitude, but don't make any factual claim (e.g. "Dr. X is the worst human being ever, his staff are terrible"). For instance, from Vogel v. Felice, defendant called plaintiff one of the top 10 dumbasses. The court rejected this as a defamatory statement because
in order to support a defamation claim, the challenged statement must
be found to convey “a provably false factual assertion.” (Moyer v.
Amador Valley J. Union High School Dist. (1990) 225 Cal.App.3d
720). A statement that the plaintiff is a “Dumb Ass,” even first among
“Dumb Asses,” communicates no factual proposition susceptible of proof
or refutation. It is true that “dumb” by itself can convey the
relatively concrete meaning “lacking in intelligence.” Even so,
depending on context, it may convey a lack less of objectively
assayable mental function than of such imponderable and debatable
virtues as judgment or wisdom. To call a man “dumb” often means no
more than to call him a “fool.” One man’s fool may be another’s
savant. Indeed, a corollary of Lincoln’s famous aphorism is that every
person is a fool some of the time.
Here defendant did not use “dumb” in isolation, but as part of the
idiomatic phrase, “dumb ass.” When applied to a whole human being, the
term “ass” is a general expression of contempt essentially devoid of
factual content. Adding the word “dumb” merely converts “contemptible
person” to “contemptible fool.” Plaintiffs were justifiably insulted
by this epithet, but they failed entirely to show how it could be
found to convey a provable factual proposition.
[emphasis added]. Calling someone terrible or the worst human, etc. does not state or imply a verifiable proposition.
It is very easy to cross the line and say something that is untrue. In the case of Bentley Reserve v. Papalios, defendant posted negative statements about an apartment building, saying...
Sadly, the Building is (newly) owned and occupied by a sociopathic
narcissist—who celebrates making the lives of tenants hell. Of the
16 mostly-long-term tenants who lived in the Building when the new
owners moved in, the new owners' noise, intrusions, and other
abhorrent behaviors (likely) contributed to the death of three tenants
[redacted], and the departure of eight more ([redacted]) in very short
order. Notice how they cleared-out all the upper-floor units, so
they could charge higher rents? They have sought evictions of 6 of
those long-term tenants, even though rent was paid-in-full, and those
tenants bothered nobody. And what they did to evict [redacted], who
put many of tens of thousands of dollars into their unit, was horrific
Hyperbolic statements about being a sociopathic narcissist, celebrating making the lives of tenants hell, and claims of engaging in other abhorrent behaviors are not statements of fact. (However, you could accuse a person of having been diagnosed to be a sociopath, and that would be defamatory; you could say something like "As a professional psychiatrist, I find his behavior to be sociopathic", and that would get you sued since it implies a non-existent professional diagnosis). Saying that plaintiff "(likely) contributed to death" is defamatory (slapping on "likely", again, does not sanitize a defamatory statement). Of course, if you can establish that plaintiff did contribute to someone's death, then it's a true statement, not a defamatory one.
The case of Dietz v. Perez is also of some interest, in part because the details are easily available (e.g. the complaint, various exhibits including the online reviews); eventually this went to trial and the jury found that parties had defamed each other, so no damages were awarded.
This is in the context of US law, where the First Amendment provides substantial protection of a person's right to say what they want.