This is a hard question as there's no better Spell to use in ARAM; all of them are useful. Clarity is very useful, and I always recommend it, and Heal is a must there. Also, you have to be sure you always use Flash there unless you are playing Shaco or Ezreal (in certain cases).  It isn't about the best Spell to use in ARAM. Instead, it's a combination that your team has to agree with to have a balanced Summoner Spell combo.