I personally not really care what the top speed of a motorcycle is, sure it's helpful to know which motorcycle has the biggest punch to get away from something... but I prefer to look at the comfortable cruising speed of a motorcycle...
With the three Yamaha motorcycles the comfortable cruising speed is very very similar, the Yamaha Exciter 150 (being the less weight to engine power) is probably the fastest at the comfortable cruising speed, the YZF-R15 is likely the second, and the Yamaha Spark 135i is probably the last.
But the Yamaha YZF-R15 will very likely be the better bike at higher speeds, the Yamaha Exciter 150 and Spark 135i will compete very very close in tight and busy Bangkok traffic.
It's clear That Yamaha Thailand looked at the success of the combo of the Honda Sonic 125 and the Honda CBR150R, and copied that concept with modern alternatives....
Which Yamaha motorcycle is faster? Is it the Yamaha Spark 135i, the Yamaha Exciter 150 or the Yamaha YZF-R15?