I use a MacBook Pro 15" Retina mid-2014 as my main laptop. The screen is beautiful. I upgraded it with a 2TB SSD (Apple OEM from a Mac Pro, but Intel 660P works with an adaptor), and an Adam mDrive SD adaptor (currently 400GB, but hopefully 1 TB or 2TB eventually).
Recently I bought some more Thunderbolt 2 adaptors (they're getting hard to find). A dual USB and Ethernet adaptor has been very useful, I didn't realise how often I need 3 USB ports. The Transcend JetDrive 855 is also great for using the old SSD over Thunderbolt, and also supports M.2 using an adaptor.
Earlier this year my spare laptop was a 2013 MacBook Air 13". My dad's MacBook Pro 13" mid-2015 had a swollen battery, so he bought a new laptop and told me I could inherit his old one if I repair it. So I gave the Air to my girlfriend. The reflective coating is peeling off the 2015 Pro's screen, and that makes it look worse. But the battery life is still much better than my 15", for reasons I don't totally understand but is probably related to background processes.
About a month ago, I repaired a friend's Mac SE, and he gave me his 15" MacBook Pro 2007! That was the computer I had when I went off to university. Oh, how I love that keyboard. It feels amazing. The keys blend together without the holes in between, it's a joy to type on. Now I'm upgrading it with a dual USB 3.0 ExpressCard 34 adaptor and a new battery, and I'm wondering about buying a 4 TB Samsung 860 EVO (or two, one for the optical bay). That would give it more capacity than my main laptop!
I'm still procrastinating the SSD upgrade, because I wonder if new 2.5" SATA drives will be released with even more capacity, e.g. 6TB or 8TB. It seems like Samsung often schedule product releases in November, so if I haven't heard by the end of the month, I'll probably just order it. If a new 4 TB M.2 drive comes out then that would be nice too, for my main laptop. There's also the possibility of a dual M.2 to SATA adaptor, but then the M.2 drives need to support SATA and I think the 2TB Intel 660P doesn't.