Biggest revenue stream depends on time, current job aspects, and who you're asking.
For me, at the beginning of this year most of it was collectively from tips and tapas ad revenue. Then during summer the most important income source was my patreon. My patreon lost a lot of supporters in autumn until just recently when I ran it through some huge improvements, causing the tide to turn, so in autumn it's been a combo of webtoons pledge program and my premium comic. Heading into winter it looks like my patreon will grow, webtoons support continues to be a good add, premium income is improving gradually, and since i got a tipping reward system installed that is on the rise too. I also occasionally get a small boost through book and pdf sales. Basically all of my main income sources are going up right now it seems.
Many comic artists who put all their eggs in one basket ended up screwed royally. One good example is Inkblazers, which went down pretty fast when investors lost interest. Because inkblazers had very little income source besides the investors money, the entire site structure crashed as soon as the investors ditched. Many of the artists hired by Inkblazers where thus safe in one moment, and in the very next they stood bare feet without a job and with very little time to build up new income sources.
Moral of the story is: when in the creative field, unless you're earning wages that are large enough to let you save money for a rainy day, don't put all your eggs in one basket. When you have a good month, try to save money in case a bad month is on the way.