The best stage is the second stage in my opinion (you run around on land and make allies or kill other species; you can also get a few member to run after you - try to get "Rogue" unites as members for that team, because they are really strong).
It´s easy to get all evolution options in stage 1 and I would get them all before going to stage 2.
Stage 2 is my favourite (I said that before

)
Stage 3 is quite short; make friends or kill the other 4-5 tribes (I made friends with them very quickly)
Stage 4 is even much shorter, I was a religious tribe (mostly plant-eating in the previous stages) and took over the neighbour cities until I got enough money to buy the rest of the world (you need to get a trader-nation-city for that option; it has cost me 12k per city left I think and there are 12 cities on the planet; I had 8 cities when I had all the money because I bought many planes).
Stage 5 is also funny, but complicated in the beginning and the stupid events that are coming every 1-2 minutes (it feels like 1-2 minutes, but is not that fast of course) can be a pain in the ass. I tried to wipe out another nation and had to fly back to my planets to fend off pirates or stuff like that between every single planet I took over (sometimes I was able to get 2 before I had to fly back). I even needed about 5-6 attacks to get their home planet because of that

I was a "friendly" race, but I had 2 races close to me that were agressive warriors and always wanted to have more and more money (a little advice: pay it, because a 2 vs 1 in the beginning sucks if you are no warrior and try to colonize).
It needed a while, but I was able to wipe both of them out (I payed one and battled the other one); later I changed my special skill (depends on the style you played in stage 1-4) to the trader skill and got 99.999.999 $ (the maximum) and 30+ solar systems.
I´m not officially done, but I stopped playing there (I went a little further, but I don´t want to tell the story).

the game is nice, but mostly the same. Stage 1 is funny in the beginning (the first 2 times played), 3+4 are boring or done too easily, 2 is a lot of fun and 5 got the longest motivation if you understand how to play that stage (you spend easily 20+ hours before having a huge empire, but you´ll have to repeat many steps hundrets of times while doing things that you truly try to do).