Hi Ali!
Your sentences are all grammatically correct. I guess a couple of things will help you determine which one is best:
1- the context: which expression is more relevant to what you are talking about? Your subject is "I"; therefore how does "I" usually speak?
2- level of language: somehow connected to the context too. What do you want to sound like? Formal, informal? Who are you talking to? Your best friend, your boss? This for sure will determine which expression you will pick.
Regarding the use of "coincidental", it means "happening by coincidence / the result of coincidence". If you really wish to use this word/form, I suggest you use it on its own: "I ran into an old acquaintance today. It was totally coincidental."
So no real answer here for you, except the above guidelines which you can apply to anything you say/write in English.
Hope this helps! If you wish to discuss in more depth colloquialism and expressions, feel free to visit my teaching page and get in touch!
Micha.