I've found Giant to be good, but a lot of agents have a list of 'preferred' umbrellas they like you to go with (often, they have systems in place that simplify the whole timesheet/invoicing process) I would seriously advise looking into setting up your own limited company though, Doing so will allow you to pay yourself through share dividends rather than a salary (apart from a small salary, within the tax allowance, that most people prefer to pay themselves) which saves a noticeable amount in tax/NI (assuming your contract doesn't fall within IR35) Its fairly simple to set a company up and even more so if you go through an accountant (mine costs £125 per month but looks to be able to save me approx £12,000 per year)
Pedis ex oris