Unicron was pretty neat for that time (after having fully rounded planets like Primus and Darth Vader, it just looks like a flat tire). Starscream was one of my favourites, and was recoloured at least four times by Hasbro.
Demolishor and Megatron were good in their alt-modes, but had some flaws.
HotShot and Cyclonus were flawed in many ways. So were Blurr, Smokescreen, Sideswipe, Hoist and Thrust.
Red Alert had so many accessories, it was like a Gen1 toy.
Jetfire was pretty good, without too much being sacrificed for its combiner mode.
Tidalwave was also very good, but the Hasbro colours were odd with no explanation.
Scavenger was an odd one - it was an Autobot, but clearly homaged a Constructicon (in both name and colours). The gimmick turned it into a brick too.
Optimus (superpants) was well thought out, with its auto-transforming feature.... and having the ability to mix and match with combiner elements (something that Energon would later feature as its own gimmick).
The Minicons were quite impressively engineered figures for their size. And each 3-pack had a theme or gimmick, like combining.
It was a shame that the Dark Saber wasn't translucent like in the product images... which was always a tease for me.
It was probably the only redeco I felt was necessary too, based on the story.
Laserbeak was another tease, as the promo images had it coloured black like a cam-corder, but just before it went into production, the colours were changed to fluro orange, due to it's (very vague) gun mode.
Armada gave us a few new interesting characters and concepts...
Wheeljack - An Autobot who joined the Decepticons. (the name was given to a Decepticon because the designer at the time - Aaron Archer - thought it sounded too evil to be an Autobot name).
Sideways - A headmaster with two heads - one for each allegiance, as he was a doublespy.
Overload - Technically was also a Headmaster, as the main robot was basically an exosuit for the Minicon Rollout. I'm not even sure if Overload was sentient to begin with.
Beast Wars recolours - towards the end of the line, several Beast Wars toys were redecoed into this line, with Minicons, because the series was so popular and Retailers wanted more product. This also led to the concept of the "obligatory redeco", that has become another legacy of Armada to this day (to pad out a line, and to reduce the cost of each mould).
(Can you tell that Armada was certainly one of my favourites...)