This may save you some money.
CHARGING SPEEDS
The USB-C cable that you use to charge your phone has no affect on charging speeds, because the minimum power delivery of a USB-C cable far exceeds what the iPhone 15 can accept. Officially, all four of the new iPhone 15 models take 20W. Unofficially, it may actually be 27W-35W depending on which source you're reading. All compliant USB-C cables will deliver 60W at minimum.
What matters is the USB-C power adapter you're using at the other end of the USB-C cable. A 35W adapter will cover all possible bases, ensuring that your phone charges as fast as possible. If you have power adapters laying around that deliver even more power, those are fine too. Your iPhone 15 will treat a 240W power adapter exactly the same way as it does a 35W power adapter. It will only draw what it can use.
DATA TRANSFER SPEEDS
The iPhone 15 and iPhone 15 Plus use USB 2.0 for their USB-C port which has a data transfer limit of 480Mbps. There's no such thing as a modern USB-C cable that will transfer less than that, so any USB-C cable will do.
The iPhone 15 Pro and iPhone 15 Pro Max use USB 3.2 Gen 2 for their USB-C port which has a data transfer limit of 10Gbps. You will have to purchase a USB-C cable that's capable of transferring 10Gbps separately in order to take advantage of this, as the one packaged with the phone is USB 2.0 which will limit you to 480Mbps. Apple does not sell one. The next step up from their USB-C USB 2.0 cables is Thunderbolt. Speaking of which...
THUNDERBOLT
Unless you have another use for a Thunderbolt cable, there's no reason to purchase one for your iPhone 15 for charging or transferring data. It offers no benefits, as the power delivery (100W) and data transfer speeds (40Gbps) delivered by Thunderbolt 3 and Thunderbolt 4 far exceed what the iPhone 15 is capable of accepting. Thunderbolt will not charge the battery any faster or transfer speeds any faster on an iPhone 15.
CHARGING SPEEDS
The USB-C cable that you use to charge your phone has no affect on charging speeds, because the minimum power delivery of a USB-C cable far exceeds what the iPhone 15 can accept. Officially, all four of the new iPhone 15 models take 20W. Unofficially, it may actually be 27W-35W depending on which source you're reading. All compliant USB-C cables will deliver 60W at minimum.
What matters is the USB-C power adapter you're using at the other end of the USB-C cable. A 35W adapter will cover all possible bases, ensuring that your phone charges as fast as possible. If you have power adapters laying around that deliver even more power, those are fine too. Your iPhone 15 will treat a 240W power adapter exactly the same way as it does a 35W power adapter. It will only draw what it can use.
DATA TRANSFER SPEEDS
The iPhone 15 and iPhone 15 Plus use USB 2.0 for their USB-C port which has a data transfer limit of 480Mbps. There's no such thing as a modern USB-C cable that will transfer less than that, so any USB-C cable will do.
The iPhone 15 Pro and iPhone 15 Pro Max use USB 3.2 Gen 2 for their USB-C port which has a data transfer limit of 10Gbps. You will have to purchase a USB-C cable that's capable of transferring 10Gbps separately in order to take advantage of this, as the one packaged with the phone is USB 2.0 which will limit you to 480Mbps. Apple does not sell one. The next step up from their USB-C USB 2.0 cables is Thunderbolt. Speaking of which...
THUNDERBOLT
Unless you have another use for a Thunderbolt cable, there's no reason to purchase one for your iPhone 15 for charging or transferring data. It offers no benefits, as the power delivery (100W) and data transfer speeds (40Gbps) delivered by Thunderbolt 3 and Thunderbolt 4 far exceed what the iPhone 15 is capable of accepting. Thunderbolt will not charge the battery any faster or transfer speeds any faster on an iPhone 15.
