At the moment the parties of “comma” and “no commas” reached a 50/50 balance, and I think it’s a good time to relay the official grammarians’ opinion on the matter.
Comma! Because it falls into the rule of “direct address“, therefore should be punctuated. So if you want to write me a formal business letter the proper way is to start with “Hi, Tat ” (followed either by : or another comma). If you’re not so formally inclined, though, I will not object to a “Hi Tat” greeting. Unlike someone at work who saw fit to correct my email to a vendor – and they were wrong. Yay, there is nothing like feeling vindicated!