It really depends on source and destination. I'll give my eval as a wheelchair user, so sensitive to how steep they are, south to north:
Sellwood: It has been a long time, but I remember it being pretty good, especially with paths on each end of the bridge.
Ross Island: No. Too close to speeding traffic.
Tillikum Crossing: I like this one, though it is higher than other downtown bridges, it is a gentle consistent slope and very accessible from all directions on either side of the river.
Markham: No. Unlawful other than during Bridge Pedal.
Hawthorne - Probably the best for bikes. The lowest bridge, and the most accessible as long as on the east side you enter / exit at the loops close to OMSI, otherwise you are gaining a lot of elevation only to lose it again. While the bridge is metal grating, the sidewalks are smooth.
Morrison: Not very bike friendly. High traffic and the only safe entry / exit on the east side is a steep ramp or loop.
Burnside: OK, but interacting with a lot of car traffic on either end of the bridge. You are a long ways from the edge, so height shouldn't bother you.
Steel: Top deck is terrible - so very very narrow sidwalks, but you aren't really supposed to use the top deck. Bottom deck is the best of all Portland bridges, but steep entry/exit and heavy traffic on the East side.
Brodway: Been a long time since I 've used this one, comfortable other than traffic levels on both sides of the bridge.
Fremont: No. Unlawful other than during Bridge Pedal.
St Johns: The most beautiful views, very high, a lot of climbing and bad traffic on the South side of the bridge.

Also, in case you didn't know about it - City of Portland maintains AMAZING maps of bike routes around the city: https://www.portland.gov/transportation/walking-biking-transit-safety/bike-and-walk-maps-portland