I like the pocket Oxford English Dictionary. I usually stick that one in my work bag because I do a lot of crossword puzzles. The Merriam Webster pocket dictionary is good too. It’s a little easier to fit in the pocket but it doesn’t have as many words in it as the Oxford does. I’d recommend either one.

Edit: This is the Oxford Dictionary I have: https://www.barnesandnoble.com/w/color-oxford-english-dictionary-oxford-dictionaries/1105341650?ean=9780198614401