Invertron
Now? Merlin, no contest (assuming it has the necessary clearances). But in the years between the planned Merlin ISD and the acheived ISD there was no choice. Everyone talks about ISD slippage, but it is very difficult to manage the complementary OSD extensions. I think those involved in Sea King ASW in the late 80s - to date did a fantastic job given that on any given day they were being told "no more cash for upgrades, Merlin's round the corner". By 1991 funding for avionics and build standard was restricted to safety issues only. In 1992 the RAF (by this time in control of RN support funding) even ditched safety. (Suppliers - They were ignored). So, when Merlin finally appeared people compared it with the SK they flew last week, but actually that SK had a build standard that was more or less frozen 15 years ago. There were a few exceptions - it is no co-incidence that the development continued on the Sonics, for example, so as to avoid gapping the availability of expertise and losing momentum. And of course SK was the first aircraft / system to benefit from Differential GPS, albeit at OEU Boscombe Down. This was the single most cost-effective device in sonics development for decades, as it allowed the MoD to tie the company down for the first time on accuracy/ranging issues, which hitherto they had got away with as we had to rely on Radar / Doppler / GMC for positioning info during trials - all of which handed large tolerances to them on a plate. Good times.