G1 ran for 302 issues. (just sayin')
For the best references just read the wiki or the "-tions" stories they are at best vaguely referenced to. this includes maximum dinobots. Same for Autocracy and Megatron Origins. You can do the same for the Bumblebee and Ironhide minis as well.
If you can get them on sale via comixology or borrow them, read all hail Megatron and the first 5 trades of the Costa led ongoing. They are not worth paying for.
Read or purchase chaos theory and police action as they most directly precede 125- death of Optimus Prime. Last stand of the Wreckers is not massively important but is an excellent comic series by a similar creative team.
As for the spotlights from the Furman era, some are very important to the current stories, others worthless. You may want to read: Shockwave, Hotrod, Wheelie, Nightbeat, Arcee, Hardhead, Cyclonus, Optimus Prime (not to be confused with Orion Pax). I could recommend another half dozen spotlights, but those are the ones that jump most readily to mind regarding current stories, especially Arcee, Shockwave and Wheelie.
Happy reading.
Looking For: Wreckers Saga TPB Collection (with Requiem)
There's a series of 8 collections (handily called Transformers: IDW Collection for about $40 each online) that includes everything you could have read up to the MTMTE and mostly in order.
But it doesn't mean you should have or must have. Generally MTMTE works as a standalone comic (with the 125th special - hence why it's included in the MTMTE collections).
The two issues I think are must reads are James Roberts issues of the IDW ongoing series (#22 and #23). They most involve flashbacks set in the prewar era and introduce many of the elements (like functionalism) that he explored in further in the Shadowplay 3-parter.
The most recent MTMTE story ties into Last Stand of the Wreckers, and that's also a good, and generally standalone, series worth reading.
RID on the otherhand is a bit more tied to things like Spotlight:Arcee, and Chaos, and All Hail Megatron.
Yeah there were over 300 UK issues, but they had on average 11 new pages per issue (at most 17 pages and that was rarely) compared to the average 22 pages of an IDW comic. But then 80s comics weren't decompressed so perhaps more happened per issue. It's probably somewhere about a tie, but IDW is still ongoing.