Well, my best was definitely signing a 31 year old Philip Hughes on a free at the start of my third season in the expanded League of Ireland save I have. I reckoned I might get one good season out of him and be glad of that. Despite his age, he powered me towards the First Division title and promotion with 22 goals. I convinced him to be a player coach and thought I'd have him as coach and back up. Two seasons later he is still banging them in and generally first choice striker, while improving my coaching set up dramatically.
After that, my best signing came towards the end of the midseason transfer window in my first season in the Premier. We were absolutely dying and slowly driftingfurther into relegation trouble. To my surprise I was able to land Sean Murray - a current under 21 international midfielder - to my rather unfashionable side. With him pulling the strings, we slowly picked up enough points to make the relegation play-off, which we won. If I recall correctly, his first goal of the season started us off with a win in that playoff series. He stands out head and shoulders above the rest of the middle, the only problem is that if he is out, we really struggle. I'll have to battle to get a worthy central foil to him!
On the otehr side of things, almost all transfers seem to be freebies in the League of Ireland owing to financial constraints. However, the past two seasons, I managed to sell players for actual money - the only transfers in the Premier each season! One was a decent keeper who wasn't getting any playing time - simply because I didn't like him I confess. Most impressively, I sold a defender who I picked up to shore up my extremely young defence in the premier, but I had come to call The Jinx, as he always had crappy performance stats and seemed to be a guarantee of a defeat. Drogheda, the fools, paid a grand for him! I'd nearly have given hem that to take him away!
I think that this is what the original poster expected out of the thread, not a guide to good transfer targets, or a list of big time successes, rather things that pleased the writer, and a little colour to the stories.
Has to be Pablo Guinazu 3.3million beginning of 3rd season for Real Sociedad. Although fairly old, his experience and sheer class inspired my team to an unexpected first La Liga title in 30+ years. Guy becomes a great coach too. Really one of my favourite early-veterans.