What is the best cell phone plan that has unlimited data and a decent price?
As others have mentioned, there is no such thing as unlimited data on a cellular smartphone plan, so you should come up with a way of quantifying your actual usage. As of 2017 in the US, cellular operators may offer 22–30 GB of monthly usage, marketed as "unlimited" with the expectation that the vast majority of subscribers won't manage to consume so much in a month. Subscribers exceeding this non-marketed cap are generally subject to throttling down to minimal bandwidth, although Verizon has been accused of throttling subscribers' video streams within the cap allowance. Other operators, like Sprint, may offer plans with media-specific throttling (480p video, 500-kbps music, 2-Mbps cloud gaming, etc.).
The only truly unlimited plans appear to be IoT/M2M plans intended for networked machines, not humans. T-Mobile's LTE Cat 1 plan offers unlimited data at 64 kbps (enough for a few VoIP calls) at $20 per year per device with a Sequans LTE Cat 1 module. If you know much about circuit boards, you could presumably get that plan, but you'll need to build an interface for connecting other devices to it. It's not really for consumers.
Among the smartphone plans marketed as "unlimited," there is Sprint subsidiary Boost Mobile's $50 prepaid plan with throttled videos, games, and music; Sprint's $55 postpaid plan with throttled videos, games, and music; AT&T's 3-Mbps-throttled "Unlimited Choice" plan for $60; and a number of more expensive plans from T-Mobile and Verizon. (But if you're 55 or older, T-Mobile offers its "unlimited" plan for $60 with taxes and fees included.) These plans may be differentiated by their hotspot/tethering policies and international roaming, so take that into account when shopping. Also, keep in mind that only 66% of downloads on Sprint's network complete at rates above 5 Mbps, so Verizon, AT&T, and T-Mobile (in that order) would be better choices for average bandwidth, latency, and reliability.
What is wrong with people. When I worked, I was the resident PC expert and people came to me with their problems. One day, a gentleman came to me and said: "My computer is hosed. How do I fix it?" I asked for more details and his face went red and he said: "I told you all I need to. Are you going to give me an answer?" I said that I didn't even know what he meant by "hosed" and he shouted at me that he needed an immediate answer and stormed away. Why do people think others can read their mind? If you're in the USA and Sprint has good coverage where you intend to use the phone, I'd say Boost at $50/month. If you are in India, I haven't a clue.
Every carrier in America offering, "unlimited data" also has what's called a Reasonable Usage Policy CAP. This is a legal loophole that most feel should be closed by the FTC. Unlimited Data is only unlimited until... you hit your reasonable usage policy cap limit.
This is tricky marketing that is not honest or ethical. Great question!
For what country? The UK, India, The USA, Canada, which? I really don't understand why people keep posting such vague questions, expecting an answer. People use Quora all over the world in English, so please keep it in mind next time you post.