From my childhood, probably opening up a complete Devastator set. More recently, my most pleasant experience were toys that weren't even meant for me. I have two cousins and decided to load up on Animated Figures for them. From Toys r Us, they both got Leader Bulkhead w/Activators Bumblebee, Voyagers of Prime and Grimlock, Deluxe Soundwave and Activators Grimlock. Since they didn't have two of Megatron, one got Activators Megatron and the other Activators Thundercracker. I helped one cousin unwrap all that stuff and they liked them. I was happy.
For me it was way back in '86....I walked into a local toy store and lo and behold, I saw G1 Optimus! The following week or month later it was G1 Megatron!
Probally either when the first movie came out. I had purchased a classic BB and barricade a couple weeks prior. Then I went to work to pick up my check, saw the voyager prime and megatron. Purchased about 7 or 8 there and then went to TRU and got some of the origional voyagers. Other time was with animated. I had only collected a select few. I had been waiting for blitzwing to come out. Again going into work to get my paycheck, I saw him, cashed it. Picked up Blitzwing, Optimus, Lugnut, Grimlock, Lockdown, Bumblebee, Prowl and Leader Bulkhead. Had also picked up Transformers on Blu-Ray. Good times. Probally be a new one this summer at botcon.
Tough one. It's between opening Alt. Silverstreak (arguably the toy that got me into collecting in the first place)... The Cybertron toy release (I'd been eyeballing C. Hot Shot for awhile... was so excited, he got opened right in the parking lot), or walking out of Walmart with the only Alt. Mirage I've ever seen. Of course, seeing a restocked shelf with all the newest stuff is also pretty awesome, no matter who you are as a toy collector.
"Here, i had found these sitting in my basement from my childhood, they are yours." (various friends said this about Complete G1 Optimus Prime Complete G1 Shockwave Nearly complete Scorponok (Lord Zarak included) Predaking - 5 bots, fists, feet, head, sword
As of recently? Having no problems anytime I went to get a "so called" hard to find bot... specifically Universe Ironhide (got 2 on 2 completely different days) and rows of Hounds and Cyclonuses.