In college, we used Mounce’s Basics of Biblical Greek. In seminary, we used Baugh’s First John Reader and his Primer. I enjoyed these grammars, learned much from them, and still value them (although somehow, somewhere, I lost Mounce; I need to get another copy!).
In order to keep up my Greek I recently purchased David Alan Black’s Learn to Read New Testament Greek. It is a bit different from the other grammars I’ve used, but so far it’s been good to go through. The Greek font is very readable, Black’s explanations are clear, and I like how he gives vocab helps by showing English words that correspond to Greek words (i.e. swma – body – somatic; kardia – heart – cardiac, etc.). I also like the exercises at the end of each chapter – so far they fit the chapter well. If you’re looking to learn Greek, this one would be good for that, but there is a lot of discourse – it gets wordy sometimes. I don’t mind it now, but it could have been too much for me when I was starting to learn Greek. FYI: I read through a chapter a week (give or take), which is a good pace so far.
Anyway, if you’re looking to keep your Greek up, I recommend getting another Greek grammar that you can work through and review the concepts and vocab. So far, Black’s grammar has been helpful! Feel free to suggest others if you have favorites.