It's modern, so apologies, but the current crop of dual pull/push shifters. Until I went back to standard shifters I didn't realise how convenient and useful the ability/option to push a shift lever both ways to achieve the same shift direction actually was.
On the retro side of things, hmm. M900 shifters aren't that great, but having recently been out and ridden for (to me) a considerable distance, I'll nominate 3 items:
1. My DBR AXIS TT ti frame. My first ti frame and it rides beautifully. Very comfy yet responsive. Titanium is a lovely material, that is until I build and ride my yo eddy, then I'll have some steel to compare it to.
2. Specialised umma gumma team and pro control tyres. Lovely and grey for starters but grip in abundance. It took me a while to get used to the fact I could turn at the point I needed to, not in advance.
3. Aside from the silly proprietary rings, xtr m952 is simply excellent in all departments in my view, but if I had to pick a part, it's the satellite rear shifter for bar end fitting. A simple idea, and one that until you've tried it, you've never realised how useful it can be, especially on a long ride with plenty of uphill sections, or simply long flat cruises.