Well not only is Goh Chok Tong a lousy orator, I think he's also a lousy politician. There were 4 candidates to be Singapore's second prime minister - Dhanabalan, Tony Tan, Ong Teng Cheong were the others. And of the four, he's the lousiest, but he got the job because Tony Tan didn't want to be a seat warmer.
The 4th candidate for PM was not Dhanabalan but Lim Chee Onn but he was brought down by the NTUC and was forced to resign (or was it just an excuse?). Dhanabalan, although highly able, was just a technocrat.
Chen Show Mao is a dilemma as a speaker. In parliament and in any formal setting with a prepared script, I think he makes an excellent speaker with his broad knowledge, intellect and class from years of mixing with top level people, the captains of industry of the world.
In Mandarin he is even more into his elements, when he can do it extempore.
But in a rally, I find his ability drop a couple of rungs. He becomes a bit hesitant in places, with some extra-ordinarily long pauses and, occasionally, a problem in timing. Maybe public speaking in front of a rowdy crowd is something new to him. Pritam Singh in that respect is a better speaker and can play to the gallery.