I have all the IDW Classics except 6 which is about to be released, but what goes after that? I know the UK continued if I remember right and I have a few of the Titan TPBs but where does that all fit in? And what about the G2 comic? I just want to get all of this and read it in order. Can anyone help me out?
The marvel comics compose their own self-contained continuity. The marvel UK stuff acts as supplemental material, but more or less fits in with the other marvel stuff. The G2 stuff follows the marvel G1 stuff, and there's a few issues of GI Joe that go between the two. Depends on which Titan TPB stuff you mean, but the marvel run goes from 1-80, and g2 goes from 1-12, though the UK stuff uses a different numbering scheme. The TPBs should say what issues they cover.
You might have use for this Marvel Transformers cross-reference that I created a few years ago. It lists all the US and UK issue numbers as well as the corresponding Titan trades. It isn't the proper reading order (with two-three stories often running parallel to each other in the UK it's difficult to define a proper order; and some US stories were not published until much later), but it's close to the publication order, and that works sufficiently well.