Electric motors with both stater and rotor as electromagnets
You state you are after "powerful" and "efficient" ... what power level? 

You are right to consider machine topologies that do not involve brushes as these are inefficient, wear out but also limit maximum rotor velocity 

Mechanical power is the product of shaft torque and rotor velocity: \$P = T\omega \$ How you generate the maximum power is a system architecture point of view. Higher torque or higher speed. 

There are a number of machine topologies that do not require brushes to provide rotor magnetic flux. 


Permanent Magnet Synchronous Machine 
Switched Reluctance machine
Wound rotor, rotating rectifier synchronous machine
Squirrel cage induction machine 


One of the most power efficient topologies is the PMSM, especially when optimised and excited with AC - PMAC. The reason for this is part of your required flux generation is already provided by the magnets on the rotor. 

At some point however the use of permanent magnets is not feasible (temperature etc....)  

Switched Reluctance is another topology and these come into their own at very high speeds and environmentally harsh environments Not as power dense or efficient as a PMSM (96% compared to 91%) but still a very viable topology, chosen more due to fault tolerance or environment.  

Wound rotor, rotating diode synchronous machines are the 3rd type. A DC current is supplied to the aux windings &amp; via a rotating transformer, the main exciter receives power. AC is supplied to provide zero speed torque. This type of machines are predominantly used in power generation.

Squirrel cage induction machines.  The workhorse of industry. Rotor flux is generated via transformer action between the stator and the rotor. Reasonably poor efficiency. Field orientated COntrol can improve operating point efficiency w.r.t. variable speed control, but it still losses to PMSM