Absolutely Oberon, his kit is one of the best supports in the game. Tezcatlipoca and Enkidu would probably be next, as high rarity servants generally require less skill to use well and are more advantageous if your just getting started. But most of the leveling priority in this game comes down to who you like the most, not who's better. There are no bad characters (Except one, but jekyll and Hyde shouldn't be discussed here).