The best gps for motorcycle use is in my opinion the gpsmap276C (or the similar 278, 478). Unfortunately it is discontinued but there are a lot used units avalible in good condition and for reasonable prices, especially compared to new ones of the alternatives.
Of course there are pros and cons for different types.
You have full control over the behaviour of the 276C, i.e. you can control the auto zooming feature. This is not the case with most other units and it can drive you crazy when driving and the zooming of the nuvi or zumo has a life of its own.
If you want to enter an address and go there the nuvi550 and the zumos are better.
If you create a route in mapsource with waypoints or an offroad route with a track to follow along small roads and paths the 276C types are superior. This is the way I use my 276C most.
If you are used to the 60csx the somewhat "special" user interface with the 276C will feel familiar, pressing the small buttons with gloves while riding is normally not recommended...
A negative side of the 276C is that it uses the old, garmin special, memory cards that are small and expensive. Normally you get some of them if you buy an used unit. I manage good with two 128MB bricks and transfer the area I need before making a multi day trip down Europe.

Then of course you can have the best of both types as I have. One 267C for route following and the more advanced topics and one cheap nuvi200 if I just want to go to an address, most of the time it shows a topo map just for fun. The nuvi is not waterproof so in heavy rain I take it off and put it in my pocket even if it is behind a small windscreen.