First off, I'm assuming that you mean "that the PCs have to fight/survive" - I see no problem with having the Tarrasque stomp past in the distance at L1, but having them stuck in a cave with it would usually be a dick move.
Even then, it depends on the monster. Level 1 is so swingy and specific-character-dependent that I'd always eyeball rather than trusting CRs. Like for example, I'd happily put them against this character (optimized for tavern brawling):
Dwarf Warrior 10 (CR 9)
Stats: 11/12/17/8/10/7 (standard array, ++ to Con)
Feats: Improved Unarmed Strike, Close Quarters Fighting, Great Fortitude, Poison Healer
Combat: 85 hp, AC 11, hits at +10 for 1d3 damage.
Also can drink anyone under the table and heal himself by doing so.
But not this one, unless I wanted something extra deadly:
Bone Creature Xeph Warlock 1 (CR 1)
Feats: Mortalbane, Point Blank Shot, Psionic Shot (extra feats from taint/flaws)
Combat: 11 hp, AC 18+cover, hits at +4 (touch) for 5d6, from 250' away (Eldritch Spear)