Formal meeting will probably be considered overkill if it's going to take you 2 minutes to resolve the issue, or the issue can be resolved in response to an email.
(Messenger) chat is the least formal of the lot - purely from a formality perspective, if you have the other person as a contact on chat, presumably you can use any of the above to communicate with them (but the other factors still come into play).
Informal talk is informal.
Phone is fairly informal.
Email varies in level of formalness.
Formal meeting is obviously the most formal.
Phone causes quite a disruption.
Informal talk is equally disrupting, but it's much easier to see if someone's busy, at least with someone else.
Chat may or may not be as disrupting, depending on the program settings and how distractable the person is.
Email is not that disrupting, but it may take a while to get a response (again varying from person to person).
Formal meeting is disrupting, but, given that it's scheduled, it's less significant, but you'll need to arrange this via one of the above forms of communication (typically email).
Why is the amount of disruption important?
With some occupations (a programmer, for example), and some people more than others, it takes a while to get into something. When you distract someone, it may not be possible for them to get right back into it - they may need some time, so, while you think you're only taking up a very small amount of their time, it could end up affecting them for a non-trivial amount of time afterwards.
Will they know the answer?
If there's a chance that they'll first need to look something up or consult someone else, email or chat would be more appropriate.
How much back and forth will there be?
Email is presumably the least appropriate for plenty of back and forth (given that constructing an email takes longer than replying using any other list form of communication), while it may be the most appropriate if only a single reply is required.
In order of appropriateness for amount of back and forth (top being appropriate for little back and forth), I'd consider the means of communication to be ordered as follows:
- Informal talk
- Meeting (formal or not)
Can you express yourself well enough?
Expressing something verbally may be easier than using written text (for some people or some things), so chat or email may not be preferred.
If you think you may want to, at some point, draw something to more clearly explain something, obviously informal talk or meeting would be more appropriate.
How many parties is input required from?
If you require a bit of back and forth / criss-cross between multiple parties, a meeting (formal or not) would be most appropriate.
Phone meetings is an appropriate alternative, primarily used if the parties aren't all at the same location.
Sometimes (for whatever reason) you'll want proof of what was said.
In these cases, email, or possibly chat, would be most appropriate.