Best Blessing for +SPD -RES Saber?

It could actually matter, because you might want to team him up with a def blessing legendary hero, but that hero might be a red flier, which is not really a good pairing in a team. So first, I'd say it's about the actual legendary hero that you want to team up with Saber and not much about the actual element.

With that said, Saber has potentially good allies in Lyn and Robin - Lyn because she can provide ranged support and her blessing gives attack and robin who has flying movement, a DC weapon and gives spd through her blessing.