I just finished marathoning Iron-Blooded Orphans and need more Gundam. I was late to franchise and didn't hop on until SEED/Destiny. Which I actually really enjoyed, I know the newer Gundam series can be pretty divisive. Watched 00 and didn't mind that too. Orphans was fantastic though.
Keen to get onto Unicorn. But should I be watching the OVAs or the split up series RE0096? Are there any major differences I should be aware of? Honestly the series suits my viewing style more since I don't have time to sit down and watch a long OVA very often so I would prefer to do that if I can.
Also playing with the idea of going back and watching older shows at some point. The original would be interesting and I've always wanted to see Wing. Do these still hold up relatively speaking?
As far as I know the there's no difference between the OVA's and the series for Unicorn other than some small art details, so if you would rather watch the show it's probably fine. Although that may just ruin the pacing depending on how much you watch at a time.
You should watch the original I think it holds up fine personally. The movie trilogy adaption of it is also available to stream on the gundaminfo youtube page until september 21 (I might be wrong about that date), and while I would rather watch the series the movies are fine.
I will say that if you want to watch Unicorn you may want to watch some of the older series (namely the original, Zeta, ZZ, and Char's Counterattack. At the very least the original and CCA) before you watch Unicorn; you don't have to but it may make it more enjoyable. That or watch Unicorn again after you watch these series.
Wing is sorta divisive. For a lot of people it was their first experience with Gundam and they perfer it, but to others it may be too different from some of the other series. Like all series it has it's issues, but I would say it's worth a watch if you're interested.