Evade Window. Even though it has many levels I saw a guide online that said only two are necessary. Almost starts to feel just a little bit like Dark Souls with the iframes and lets you do silly stuff around the monster's feet easier.
Source? I used to run evade up (now evasion window) and evade distance on my Volvidon switchaxe back in MH4U but ever since they changed the moveset in MHW I can't really get into it. I heard that the I-frames they put on SA's dodges are smaller than a normal dodge which sounds like an unnecessary nerf to me. I tried throwing Evade window 5 in a SA and even then I don't feel like I could dodge attacks as well as I used to in 4U. (EDIT: I somewhat consider evade window an offensive skill simply because it increases your uptime actually dealing damage instead of healing etc.)
As for currently? I usually just ran the meta lance sets, nothing fancy, but I absolutely must have Guard on a lance. Guard 3 is good enough most of the time, but 5 feels a lot better. I sometimes use Guard Up because I have a 2 slot gem for it but there aren't a lot of monsters with laser beam attacks in this game so it's very situational. (Being able to sit in ATKT's lava bath is nice though, if a bit risky)
I haven't put a build together for it yet but some of the new moves for sword and shield look pretty sexy (I'm a fan of attacks based on timing) so I was thinking of putting together a build with the Gobbler skill on it. I once saw a decent build someone had made but I was dumb and did not write it down, but basically he had a bunch of nice skills and then had room for one or two more gems so he put something like Grinder on, but I think gobbler would have been much nicer to speed up your buffs and healing. Naturally, Wide Range is a necessity for a support sword and shield, but I sometimes put some on my other builds when I'm not sure what else to throw on.
In terms of element/status, poison is nice since it's free DPS which you don't need to hit the monster to do. Paralasys is nice if you're playing with a group of people since it increases their DPS at the cost of your own, very nice if you have someone with a greatsword. Same thing for sleep, people used to run sleep greatswords in the older games. The meta for MHW however is just raw damage, which is has apparently been remedied in Iceborne. The great thing about raw damage being strong is that it simply works against most monsters and you don't need specific builds for it. However that is also the downside, you end up using one build for everything and don't end up using the ~14 pages of loadouts you are provided. Incidentally there are 14 weapons in the game... it'll be nice to have a full page of lance elemental loadouts once more.