Your question sounds simple but it's not so easily answered.
I've used a Palm Pilot with 2 MB upgrade, a Palm Vx and a Visor with
the OmniRemote Springboard module.
The Visor works the best although it is a bit demanding of the batteries.
I have a Visor deluxe but I think any model would work fine. If you are
only using the Visor as a remote, go for the cheapest model.
The Palm Pilot with the 2 MB upgrade actually worked very well and it is
a cost effective solution if you can find one. The problem is that the Pilots
are a few years old by now and they don't last forever. Also, you'd have to
buy both the Pilot and the upgrade card used since neither are made any
As a PDA, I love the Vx. However, it is mediocre as a remote due to the
weak IR. Others have reported good success with an IR extender.
Hope this helps.