I'll answer in the context of Xometry but also give general guidelines as I helped launch the US sheet cutting offerings:

Typically, count out plasma. Between fiber laser and waterjet, there is just so much more repeatability and precision than plasma cutting. There are some super great modern plasmas, but most cutters out there on the network are for fabrication, where the edge condition will require some secondary treatment like cleanup and welding. 

Laser is typically more precise than waterjet, but our manufacturing standards are amicable for both technologies. The thicker the material, the more likely a waterjet will be used (waterjets can cut basically anything). It's important on your end to specify critical dimensions that are tighter than the manufacturing standards for that process. Edge condition may also be a consideration and there are methods/equipment to taper-correct on deeper cuts. This may require a manual quote review, and in some cases, the tolerances are just not reasonable for "first cut results" with a sheet cutting process and may require machining (e.g., a 2.5-axis mill). Xometry has access to all these abilities, it just may require a manual review if you're asking for tolerances or feature sets beyond the norm for a sheet cutting process.