RTX 2080 Ti is the best GPU for Machine Learning / Deep Learning if... 11 GB of GPU memory is sufficient for your training needs (for many people, it is). The 2080 Ti offers the best price/performance among the Titan RTX, Tesla V100, Titan V, GTX 1080 Ti, and Titan Xp.
Titan RTX is the best GPU for Machine Learning / Deep Learning if... 11 GB of memory isn't sufficient for your training needs. However, before concluding this, try training at half-precision (16-bit). This effectively doubles your GPU memory at the cost of training accuracy. If you're already successfully training at FP16 and 11 GB still isn't enough, then choose the Titan RTX -- otherwise, go with the RTX 2080 Ti. At half-precision, the Titan RTX offers effectively 48 GB of GPU memory.
Tesla V100 is the best GPU for Machine Learning / Deep Learning if... price isn't important, you need every bit of GPU memory available, or time to market of your product is of utmost important.