The most popular choice is a two flute carbide tipped straight bit. They will have less vibration and a smooth cut. A single flute is used when cut speed is more important than finish. Fewer flutes mean improved chip clearance and a faster speed. A straight bit with shear has angled carbide flutes for a slicing rather than chopping cut which produces a cleaner edge. If a plunge cut with a flat bottom is desired, look for the plunging straight bits.
Always select the shortest length needed for the project since longer length can mean deflection and vibration. Never force the bit or use a worn or damaged bit. Always use multiple passes, especially with small diameter bits. Never cut more than half the bit diameter in a single pass.