I only have one Tiffany ring - the wide version of the 1837 in yellow gold - I love it! I only just bought it and it is perfect as an everyday ring and you can get it in rubedo and silver if you don't like gold. I also like the Tiffany notes and twist rings and they would look good either on their own or stacked. The Marrakech (spelling?) ring is also beautiful but a bit more expensive.
If the ring doesn't have to be from Tiffany have you also looked at Cartier or Bvlgari? The Trinity ring from Cartier is also a lovely everyday piece that looks good worn casually. I have the small-middle size (not the smallest but the next one up) and it looks good worn with practically anything! It combines yellow, white and rose gold too so ties in any jewellery you have in other metals nicely.