I know it goes against fashion, but All Hail Megatron was a decent entertaining read for what it was. The issue that many fans had against it was that it replaced the much prefered Furman stories, and lacked the nuance and layering that Furman's stories had.

Sadly because of this Furman's slow burn had to be wrapped up fairly quickly and the Dead Universe saga takes all of 4 issues, rather than the longer story I think he had intended. It leaves the whole thing feeling rather rushed, but was a nice swan song for his first set of stories.

All Hail Megatron is largely disliked, but it had a fantastic premise and some great G1 movie like moments. It must also be noted that the AHM Coda is responsible for providing us with the IDW Prowl that we've come to love. It also gave us Drift. But we got a really good toy out of him, and Roberts is using Drift pretty well. The AHM spotlight is kinda unessential though.

Costa's run had some great ideas in it, but he seemed to lack the capacity to write Transformers as actual people (some would say he was unable to write people as actual people, too) and his stories felt dry and insubstantial. There was no flesh to support what looked like great bone structure.

I haven't read much of Roberts or Barber's ongoings as I but the trades, but from Barber's work on the DOTM comics, he'll do a great job. Roberts is fantastic, and manages to condense so much into each page. Some of the story is a slow burn, but it appears people are happy with the parts that do that because the parts that relate to the story in that issue are fantastic.

Both Roberts and Barber have shown that they are not only keen to draw from what's come before, but do so in a way that pulls something you wouldn't even expect, but in a way that respects what's been put down by the other authors before them.

One book that people seem to have forgotten about is Transformers: Drift. It wasn't that great by itself, but it introduced us to the Knights of Cybertron, the legendary ones that the Lost Light is searching for.

Short answer - well worth picking up the mainline comics
Infiltration - Stormbringer - Escalation - Devastation - Maximum Dinobots - All Hail Megatron 1 & 2 - All Hail Megatron Coda - Transformers Drift - Transformers Ongoing 1-7

If you pick up Spotlights 1,2,3, and 4 they will add to your enjoyment of Infiltration - Maximum Dinobots.