Deltatry from Gradius Gaiden
Gomander from R-Type (Delta version's my fave)
Beacon from Gradius III/V (V's is best :p)
Crek-G from Psyvariar
Darya from X-Multiply All of Axelay's!
Bah, sod it - I could go on all day. I'm a bit obsessed with bosses!
I'd say the huuuuge robot samurai at the end of Sengoku Ace would be pretty memorable. It's a shame he's too easy...then he turns into a wimpy little vase, and when you destroy that, usually a demon will come out of it. But sometimes a Tanuki figure will pop out instead. Weird eh?
And no pickles! Oh god help you if I find pickles!
-Stewie Griffin
R-Type's Dobkeratops. He just looks cool. And it's the only boss that made me do a double take when I walked by the arcade unit when someone else was playing it. I went, "What the hell...." and I backed up and stared at it in awe.
Then I about freaked when I was told it was the level 1 boss.