Use whatever kind of sight you'd use as a backup to your optic. Kindof a waste to practice with something you won't be carrying, ya?
If you want a folding BUIS, the Troy, ARMS, Magpul Pro, KAC or MaTech will serve you well.(Though I am loathe to support Troy Industries in any way, their battle sights are good kit. Get a BCM one or find a used one on the E&E for cheap since more than a few people want nothing to do with them)
If you intend to run fixed BUIS behind your optic, the LMT, DD, MI,(Pretty much a copy of the DD) LT and, believe it or not, Rock River ones are pretty good. For a fixed A2 style, I prefer the RR over the LMT. Hate the big-ass knob that gets in the way. RR doesn't have that, instead there are two low profile, hex head cross bolts. Perfect if you don't intent to remove the thing, or if you just like the extra peace of mind. Personally, I wouldn't bother with an A2 style rear. They just tend to have more things that can break, since you likely won't be shooting irons at distances where the extra adjustments would be of use.
Now if you don't want to spend the money on any of these, the Magpul will do just fine.