Conks, welcome to the forum
You've highlighted 4 seven manufacturers from different ends of the spectrum, Quantum being more of the top end budget, RH and MK being more the budget end with Tiger being both. As such I would also include the DJ Rush, Westfield and all the other locost style 7's unless you left them out of purpose.
I don't think you will find any of them particular difficult to build, really depends on your apptitude, commitment and desire to complete it.
As for the best manufacturer, well I guess you're not going to go far wrong with any of them but bear in mind RH is currently up for sale so it's future is unknown.
I'm sure you'll get more answers from people that have been there and done it with regard researching and building 7's so I'll let them get a word in.
Cheers, John
Madabout-kitcars.com