As a frequent flyer, I generally feel safe in 737s, especially compared to the A319 and A320, which just don't seem solid to me, or MD-80s, which are outright flexible (just sit with your shoulder against the wall near an emergency exit and you'll see).
But the single scariest airplane I've been on was an ancient 737 flown by Aloha Air, just last year. This was an original-issue model with the engines tucked under the wings without pylons. It was of the same vintage as the one that went "convertible" off Hawaii many years ago due to age, salt-air corrosion and the stress of countless island-hopping decompressions. This one had riveted reinforcement plates running down the full length of the fuselage to avoid the same fate. Not confidence-inspiring. Maybe Air Sibera runs older, more run-down planes than that one. But it got us from island to island in one piece. Keeps on ticking, indeed.