As a proud Yorkshireman, I chose the highest summit in Yorkshire for my Mountain Goat activation - Whernside.
I also chose Whernside, G/NP-004, because from the back it only takes around 45 minutes of walking to reach the top. I did a crazy thing of constructing my entire station actually at the summit, by constructing, I mean literally; with a soldering iron! I needed to allow enough time to build my gear and activate.
I think my favourite hill is Ingleborough G/NP-005 and I chose that hill for my 1000 summit activator points activation (true Goat, bonus points not included). I only built the transmitter on that occasion, but I made it a bit more difficult by doing my own design and using surface mount components.
home brew transmitter constructed on Ingleborough with gas soldering iron
I think the choice of summit can be made on any arbitrary rule you choose - but the most important thing is to make it fun I think!