I will update my answer to this question as my recommendations have changed since the last time this was asked. A gate at the top of the stairs needs to be very secure yet easy to operate. It is also essential that it be a hardware mounted gate and not a pressure gate. Finally, the gate should not have a threshold which could cause you to trip.
Of the gates that meet the above criteria, my current favorite is the Angle-Mount Safeway by KidCo. It is extremely versatile and can accept mounting at any angle (hence the name). It comes in white metal (Model G22) and wood (Model G32) versions. This gate spans from about 28 to 42 inches and can reach as far as 63" with optional extensions. It is not the least expensive gate, but its quality and versatility make it a good value.
KidCo has also just introduced a new installation kit for mounting gates to banister posts without drilling into the post. The Gate Installation Kit (Model K12) can greatly simplify an otherwise tricky installation.
Although the Angle-Mount Safeway is my favorite, other viable options exist for protecting the top of stairs. The Cardinal Stairway Special (Model SS30) and Auto-Lock (Model MG15) are both excellent and among the easiest to install.
Other quality options exist for unusual solutions and new (and sometimes improved) gates are being continually introduced, so “stayed tuned". As we review the new offerings, we may find even better options in the near future.
Before you purchase a gate for the top of the stairs, check to see whether the gate can be placed across the hallway and perhaps moved up the hallway while still preventing access to the stairs. Depending upon the configuration of your home, you may be able to move the gate away from the top of the stairs, making it easier to install and operate. In doing so, it may even prevent access to an “off-limits” room or a balcony.
Whichever gate you select, remember to keep it closed. An open gate is hazardous to both children and adults alike. Also, don’t forget to prevent access to the bottom of the stairs, as crawling babies may climb the stairs part way only to fall back down.