Starting with Combiner Wars, as an adult I can buy all the TFs I never had as a kid, lot of G1 influence.

 

Hardly objective and probably bias because everything else is just a spin-off from G1 for me but I'd say the original run was ideal for me.
Was 5 (and already He-Man was passe for us) when it debuted here and by the time I saw series-3's darker episodes I was 7 and it didn't seem as corny as series 1or2 might have at that age. Which you can be quite sensitive to at that age. Its like they grew with me and were catered to that age group.
It subsequently disappeared at the age it would have for me anyway.
So like others have said 84-86 - which was 85-87 as experienced in the UK.

 

It’s cool to be a fan when there’s lots of tfs content, toys available and series.
To me, despite that, the best time would be the beginning. I like it because there’s something cool about something really new, when the concept of transformers and robots was not aaaas explored, when they were relatively scarce, and also when there was a big future ahead, where you wouldn’t know what it’s going to be.. if it will develop into something big.. and then to see, how it grows, and how many cartoons and movies being developed! And then, you can collect, and be nostalgic, and tell your kids HOW it ALL STARTED!

 

2010-2015
Say what you will about the aligned continuity but it gave us the War for Cybertron and Fall of Cybertron games as well as Transformers Prime. IDW comics too were great.

And I guess our current time with the Netflix WFC trilogy toys

 

1984-86
And
2007-2013

 

I can only speak for myself, but 84-87 years were the best for me, because I was a kid back then.

in terms of collecting, with siege and Earthrise there’s never been a better time! I love the toys!

 

2010

 

1984-1988. I was the target demographic at the greatest time in toy history. Thank you, President Reagan.

 

Best years?

All of them, as every line, everything that comes out can be a homerun or a dud. I've yet to see a line that didn't have some fun/good toys, and across all of the cartoons and movies and such, there's always something to enjoy, from a lot to a little bit.

Toy wise, I'd have to say that since the Prime wars trilogy brought so many figures that had never been done or done well, and we have so many "definitive" versions that will probably not be redone for decades, if at all ever again, it would be the better years since then.

There's a LOT of amazing figures before that, but for anyone that wanted the G1/G2 and such roster of so many figures, most collection turned a crucial path when we finally had these coming out.

 

Either the BW years or the UT. Solid, innovative designs with lots of imagination. Sure, the ideas and gimmicks sometimes fell flat, but for every Armada Sideswipe we also got a Cyclonus. And the excitement of seeing where Hasbro was taking things next year ... that's been missing for a long time. Sure, the UT cartoons were a bit lacking, but the TOYS, the toys were wonderful!

 

For the toys, right now.
To experience TF I'd say during G1, with the release of the first live action movie second.