To answer your question correctly - there is no best eudemon. There are only good and bad combinations. In general - you always must gave at least 1 attack and 1 defense type demon.
Mices and superdemons are easy to star and powerfull. However - there is a lot of players who realize it, and it will be really hard to get them starranked.
In general that means - low quality demons (e.g., WarriorAtkDef, MageAtkDef, BloodSage, VenusSage) will pawn superdemon of the same star.
However - that is not always true, but depends on other ppl playing on your server - for how long they are playing and if they are non-paypallers or pay-to-play type. Example, there is a lot of Adams, Floras and the like (supereuds, i mean) at 30+ stars in Vegas, most of them are not starranked. At the same time - my BloodPhantom is 17.7*, and it is at rank 99.
The best way how to choose your pets:
1. Choose what your combination will be (depends on character class) - you will have to use at least 1 defence pet, the rest is up to you.
2. Check pet rankings at http://eo.rank.91.com for your server

I play: Vegas
Rank: Viscount
Level: 99
Pot: above 300
Forget the rest - I'm sick updating sig every day.