My top five (in no specific order)
G1 Soundwave, primarily because of his toy. Let's face it, in the cartoon and comics, he never really DID anything other than follow orders and have some nifty props. He did have the cassette minions, but is he to be voted in based on their coolness? He does have a strong recognition factor, but honestly... is that really worth it?
G1 Grimlock, both for his comic and cartoon representations. For a while, he (and his teammates, the Dinobots) were the biggest and baddest Autobots around. Even Devastator got worried when the Dinos showed up. In the 'toon, he was amusingly 'dumb', played for humor even as he kicked tailpipe and took numbers. In the comic, he was cunning, egotistical, and COMMANDED THE AUTOBOTS FOR OVER A YEAR. He was the first 'beastformer', and a truly iconic character in the brand.
Unicron Trilogy Hot Shot, as a representative for the trio of series that truly redefined the 'third age' of Transformers (the first being G1, second as the Beast Era, and the fourth being the live-action films). He's the one character that really stands out as unique to that universe (Animated version notwithstanding), and his growth through the trilogy is what warrants his placement on this list. Worthy of induction? Probably not yet... but worthy of nomination.
Beast Wars Dinobot, as a representative of the aforementioned 'Second Age' of Transformers. Let's face it... his toy was pretty lackluster. But what he lacked in plastic, he made up for in sheer character. He was a first in the Transformers mythos - a truly honorable villain, betraying his cohorts to fight alongside the heroes... an anti-hero, a black knight. His evolution from vindictive ally to grudging companion to loyal friend is a very compelling journey... capped by the incredible episode "Code of Hero." Now ask yourself... how many Transformer Deaths have made you cry? I can think of two, and one of 'em is automatically being inducted this year...
G1 Spike Witwicky, for his unique role in the Transformers mythos. He was the first human to really connect with the giant alien robots, guiding the Autobots in how to live on this planet. It was with the help of humans that the Decepticons were defeated, time and again, and that help could never have come without a loyal ally guiding them along. Alternately, the Witwicky family could be inducted, from Sparkplug and Buster, to Carly & Daniel. Heck, even lump Sam in here, who seems to be inspired by Spike, anyway...
There are lots of characters worthy to be on this list, too (why not Devastator? Jetfire/Skyfire? Unicron? Skids & Mudfl... ok, maybe not them)... as well as several non-fictional entrants. Simon Furman, who wrote some of the core mythology we use today, or Bob Budiansky, who originated so many classic characters? The voice actors, too numerous to single out in this post, that gave us so many amazing characters?
Superstar Michiyamenotehi Funana, who... erm... well... frankly, I just want to see him come to BotCon to accept his award, and get a videotape of fan reaction...
Ok, folks, let's see your lists, and don't leave out the rationalizations!