This is like asking what is your favourite colour. Your going to get a lot of answers.There is also a lot of information anyone would need to be able to give a realistic answer, what you expect it to do, budget , time scale skill level of builder. ultimate reason (a toy or hobby or something more commercial.)I guess your thinking about an autonomous ROV - For a start personally, I would go with the Picaxe system http://www.rev-ed.co.uk/picaxe/Relatively cheap, fairly powerful and fast to develop with masses of expert support in their forum. Once you have a handle on the real requirements and scope of the project you will be in a better position to look at other options.