My favorite Mother's day gifts I got when my kids where small. Little notes completely spelled wrong (which made it even cuter) full of hearts and flowers. At this age they still thought that mama knows all and is absolutely perfect. As a mum of 2 teenage boys, I can tell you this changes fast ;)
Myself I used to buy my mother a candle for every Mothers day, Birthday and Christmas gift. And she still managed to look suprised!
I still think candles make one of the best gifts, but luckily I found there are much more options. Like these lovely cactus flower coasters. Every time your mum will put her drink on them she will think of you and smile.
Another good one is the Pestemal towel. We mothers like beautiful gifts which are practical too. And this one is. It's usable as towel, scarf or tablecloth there's no way she'll not love this gift!
A piece of jewellery is something she'll treasure. When my son bought me a bracelet back from a school trip, I felt so proud I could say "My son got me this" when people complimented me on it. I think it's a mothers-thing...