You need to start off by working out which of the assorted types of multiFX would suit you best. Basically, there's three and a half types.
First is the type where patch selection is simply by going up or down the list of patches. OK if you use the same sound all the way through a song, less so if you're changing sounds in a song. Examples are the Zoom B1 and Digitech BP200.
Second is the type where you select patches by selecting a bank initially, then select from the patches in that bank by pressing one of several footswitches. Makes it easier to select a patch at any time. Tend to be fairly rigid in the signal chain, ie. you'll get a chain which roughly speaking contains a preamp, distortion, reverb, and modulation. Examples are the Korg AX3000B and Boss GT10B.
Third is the stompbox emulator. That emulates a series of stompboxes. If you are looking to replace a series of stompboxes which you don't change the settings on much but do switch the individual stompboxes on and off, it's probably the best solution. Means you can have (eg.) a flanger, chorus, and, er, another chorus, or a number of distortions all in a single signal chain. That's the Zoom B3. I think there's some multieffects of the second type that will work in stompbox mode too.
Number three and a half is the Zoom MS30B, which works as a chain of stompboxes. You can opt to have it cycle through a series of patches as per type 1, or have three of the stompboxes in the chain set to on or off and then switch the fourth stompbox by means of the footswitch.