My favorite era to play was the Future era, and probably still is because it is the most advanced age that still has a corresponding GvG map to keep playing on once you've left the age. When the all ages map replaced all maps for new era after FE, the unintended consequence is no one can continue to enjoy playing in arctic, oceanic, virtual, Mars, Saab once they leave them, because the dominant factor on the AA map is always the current age (Venus now) and the older ages and older troops above FE become utterly useless except as cannon fodder.
Future troops (rail guns and hovers) and the overall aesthetics of the age from the sea life tower to the solar walkways, were all great looking, and the Arc and Rainforest are good GB's. Most people will say the Arc is the best GB in the game, and still others will say that's when all balance began to break. FE is also the last age that I really was able to build a city using houses, production buildings, culture buildings of that era to make my city match my age with the way it looks. Now there are so many event and special buildings, a player would have to downgrade to use anything that comes off the standard build menu. Not since Future era have I had a city that actually reflects or enjoys the design elements of that age. You can tell that the designers know this as there is barely 1 house, 1 production building, 1 culture building, 1 decoration produced for each of the space ages - and we only need those for the incessant and useless event quests that require us to "build" a house, production building, culture or deco from our current age.
Of the space ages, Mars would be my favorite age, but I don't think it's fair to even compare the space ages to the ages that came before. If there were a Mars specific GvG map to continue playing on after leaving Mars, perhaps that would be my favorite age instead. Mars troops were well designed, and it was a good age to play in with the new space mechanics, colony and ore on the maps. The more this is re-used the less enjoyable it becomes in Saab, Venus, and Jupiter.
Unfortunately, the addition of space colonies have made goods production in all the space ages and goods requirements for all things concerning "end game" players completely unbalanced with all of the ages that came before. Cities no longer have to be used to produce goods, and this leads to players wanting to group up in above-FE only guilds, which as I have warned before on other forum posts, has and will be a detriment to new players being able to play with and learn from established players. It's increasingly impractical to have lower level players in a guild with space age players. Yes, it can be done, but the half of the guild in Mars and above pays a price to allow players, especially FE and below, to play with them. The only reason arctic, oceanic, and virtual goods are not much problem for most guilds is because they had years to accumulate those treasury goods, that were not used for anything before battlegrounds came along to randomly hit the age of any/every player age in the guild with high goods demands. We still have 500K to 1 million stocked for arctic, oceanic, and virtual, even after battlegrounds has been out for 2 years. I don't suspect we'll ever run short on those goods, but we have to think hard about letting in more Iron, EMA, HMA, LMA, Colonial players.
From a game design perspective, PROGRESSIVE era is probably the best age ever made, all around, the troops are the best balanced with no one troop being a "ringer" that is used for everything while the other troops are ignored, the design looks the part, and Alcatraz and Chateau are 2 of the best GB's ever released, other than the game-breaking and essential Arc, of course.
Depending on your criteria then, FE, PE, Mars are your best 3 ages.