A lot of my small saws impressed me...I guess I don't expect much out of a small saw, so easy expectations to exceed.
Most impressive is the Echo 2511t. I know it gets a lot of positive publicity online, but I thought most of it was just fanboism. A dealer happened to have 1 left in stock, so I bought it on a whim to compliment my 201tcm. Stock, it was a little better than I expected. After I gutted the muffler, opened the deflector up, and put a non-safety 3/8lp chain on, I was really surprised. My 201 doesn't even get used that much except for big removals.
Not to downplay the 201tcm however, that's been a good little saw. The 201(particularly the M-tronic) seems to get a lot of hate online. I really can't tell the difference in weight between it and a 200t, and it has good torque for a top handle. I run a 16" on mine, so I can get pretty far down when chunking out a spar.
My mk1 550xp initially impressed me with it's power and handling. I used it a bunch and it was my go-to saw for when I needed a rear handle in the tree. I even made my own 3/4 wrap handle for it. However, random hot-start issues have kinda dampened my love for it. The other major blow to it, has been my acquisition of a ms400.
IDK if I'd say that the 400 has blown my expectations away, but it's probably my favorite all-around saw. I through a wrap handle kit on it and run either a 25" or 28" Stihl lightweight bar with skip chain on it. I climb with it when I need a rear handle. I even chose it over my various 70cc+ saws for a recent falling job. Sure I had to double cut some stuff, but it was worth it vs hiking around with my 066.
Finally I'll add my ported BB 044s to the list. I've heard bad things about the big bore kits on the 044/440s, but I did it on both of mine. I machined them for proper squish, ported them, muffler modded, and put on max-flows...they're just bad-azz worksaws now and anything less than a 32" seems pointless on them.