The OTW and Engel Type VII are the best Type VII models out there. Not cheap options by any means, the Engel boat will rush you about £1900, and the OTW with one of their modules will set you back about £1600-1700. You can purchase the OTW hull minus a module which halves the cost, and grow your own dive system, which isn't an option with the Engel sub. The Engel boat is a bit too perfect looking for my liking- bit like a scaled up Revell kit, every rivet is perfectly placed etc. The OTW boat looks like it's seen a bit of action- little bit of dishing on the hull plates etc.
The MBD hull is very cost effectove. It's the old Darnell hull, and builds into a nice model, but not as accurate as the OTW or Engel models, and will require a lot more work constructing fittings etc. than the Engel or OTW boats.
The Robbe boat builds into a nice enough model. It's ABS plastic as opposed to GRP, so not as tough, and the detail is lacking (no rivets or plate lines etc.), but can be applied by the builder. A lot of people use the Accurate Armour tower in place of the kit original, which improves the look of this boat no end. Etched decks were available at one point, but I don't think you can get them any longer.
So there we go- pays your money and takes your choice.