The cheapest unlimited data plan available right now is Mint Mobile's Unlimited Plan, which you can get for only $15/month for the first three months. This plan includes 35GB of high-speed data plus 10GB of priority hotspot data.
Mint Mobile undercut the competition so significantly that Visible lowered its prices dramatically after seeing what Mint was charging. Visible’s $25 plan is cheaper than Mint’s $30 Unlimited plan (after the promo ends), but it lacks priority data, so your speeds are throttled anytime the network is busy. With Mint, you have a comfy 35GB before you have to worry about throttled speeds.
Usually if something is cheaper than the competition, there's usually a catch. Mint Mobile does have a catch, but it's not such a big negative that we wouldn't recommend the plan. Deprioritization is baggage that comes with all Mobile Virtual Network Operator (MVNO) providers. Basically, if you're on an MVNO provider, like Mint Mobile, your data speeds can become throttled in congested areas and during peak-usage periods.
The problem with Mint Mobile is that it isn't owned by a major network. Visible Wireless, for example, is owned by Verizon, which means it will get preferential treatment over other MVNOs on Verizon's network. Mint Mobile doesn't have any strong connections to the major networks, so expect it to be even more deprioritized than your average MVNO.