All spells would be useful, because magic is magic, and magic would be awesome. However, I can admit that I have my favorites. There are some spells that I think about often and wish that I truly could perform on a daily basis. Every Harry Potter marathon just reinforces the wish that I was a witch, too! (Where's my Hogwarts letter, Hagrid? I would believe faster than Harry!!)
Disappear from one place and appear in another
Why, yes I would like to disappear from one place and appear almost simultaneously in another place. No travel time? No traffic? No commuting? No gas money? I would be one happy lady. I would pop in on friends and family on a regular basis. I would enjoy breakfast in France and dinner in Italy. The possibilities are endless.
Summoning an object to oneself
Imagine being able to swish and flick your wand for something you forgot back at the castle and have it zoom towards you. Or never having to lean precariously from the edge of the couch for something just out of reach. Accio is probably the most practical spell ever, and I would be even more lazy than I am right now if I was a wizard.
Repairing broken objects
Easily fixing all those broken things I've broken?? Yes, please!! So practical and efficient and a very magical time saver. Haven't actually broken my glasses like Harry has, but I've broken so many necklaces and bracelets, that I've wished so hard for this spell!
Unlocks and opens doors
So. This one is for purely devious reasons. I have wanted to know how to pick locks for a long time now. I mean it's a good skill to have, right? Fred and George knew how to do it! And they needed the skill for purely acceptable reasons!! I actually still want to know how to pick locks even with this spell in my repertoire...
Repels outside forces, including water
Making everything I own repel water? Everytime I walk outside in a thunderstorm, I feel like I would be chaneling my inner Percy Jackson (the irony, I'm switching fandoms). This would be particularly helpful for my hair and/or makeup states during rainy weather.
Produces a buzzing sound in the ear's of people nearby to prevent them from overhearing one's conversations
This would be super convenient when having a private conversation in any public place! Anyone who has felt like the next table over was just a bit too close will identify with wishing they could mutter Muffliato under their breath and not have to wonder about anyone overhearing. It's really just for peace of mind!