In general, you shouldn't trash talk people you don't know. A more important rule: don't trash talk your own teammates. It's counter productive.
If you are just trying to gently tilt the other team, call out the "wonderful" plays they have made. Like if a point blank ult was missed: "Nice Shot!". Maybe after a team fight you just won: "Oh man, you got me to half HP, I was worried". If you get an 5 for 0 ace, you could say something like "Flawless", or maybe a good trade would be "Worth".
Be aware, this can backfire, and once you say something, the other team may will be inclined to return the favor if/when you slip up.
Biggest rule: don't trash talk to just to say "you're bad". It's boring, not creative, and tedious. Put some effort behind those words.