If you really HAVE to do it with an all-in-one (see post regarding multi versus stompboxes), here's my quick take:
XT Live has the easiest interface. Models are good, upgradeability/flexibilty is very good, good support.
GT8 is best in terms of controllability/amount of parameters, but consequently has the steepest learning curve. Need to buy some external switches to properly control it live.
Tonelab stuff has least amount of options, but best tone IMHO.
Listen with your ears, then go buy some stompboxes.-Eric