I think Chansey should be D-tier. It is outcalssed by the fatter blob in so many ways. Screw eviolite, I'd much rather have leftovers+decent special sttack. I often run Ice beam on my bliss to suprise gliscor. Though I might change that to flamethrower.
note: these calcs will be with a 252 def / 252 sdef calm blissey, and a 252 def / 252 sdef bold chansey. i frankly find the hp investment to be a waste on chansey when you could put those points in eviolite instead and get a boost; you need the extra special defense to take all the retarded hits in this game
you're pretty quick to underestimate chansey's bulk. it's true that blissey has leftovers and special attack with which it can run surprise elemental attacks instead of seismic toss. however chansey's bulk advantage is ridiculous. in terms of taking hits, leftovers only starts giving you a serious advantage when the damage is relatively small. let's take a look at specs politoed hydro pump in rain, which is basically the benchmark for special walls that do not resist water:
252+ SpA Choice Specs Politoed Hydro Pump vs. 4 HP / 252+ SpD Blissey in rain: 220-261 (33.74 - 40.03%) -- 34.08% chance to 3HKO
252+ SpA Choice Specs Politoed Hydro Pump vs. 4 HP / 252 SpD Eviolite Chansey in rain: 192-226 (29.9 - 35.2%) -- 20.29% chance to 3HKO
pretty close between them. here, the leftovers that blissey is carrying largely make up for any unlucky high rolls.
but now let's look at some more interesting calculations for things that supposedly break pink blobs:
252 SpA Choice Specs Latios Psyshock vs. 4 HP / 252 Def Blissey: 388-457 (59.5 - 70.09%) -- guaranteed 2HKO
252 SpA Choice Specs Latios Psyshock vs. 4 HP / 252+ Def Eviolite Chansey: 261-307 (40.65 - 47.81%) -- guaranteed 3HKO
4 Atk Life Orb Tornadus-T Superpower vs. 4 HP / 252 Def Blissey: 445-525 (68.25 - 80.52%) -- guaranteed 2HKO
0 Atk Life Orb Tornadus-T Superpower vs. 0 HP / 252+ Def Eviolite Chansey: 296-351 (46.17 - 54.75%) -- 58.98% chance to 2HKO
4 Atk Life Orb Hydreigon Superpower vs. 4 HP / 252 Def Blissey: 465-549 (71.31 - 84.2%) -- guaranteed 2HKO
0 Atk Life Orb Hydreigon Superpower vs. 0 HP / 252+ Def Eviolite Chansey: 309-367 (48.2 - 57.25%) -- 91.8% chance to 2HKO
252 SpA Keldeo Secret Sword vs. 4 HP / 252 Def Blissey: 548-648 (84.04 - 99.38%) -- guaranteed 2HKO
252 SpA Keldeo Secret Sword vs. 0 HP / 252+ Def Eviolite Chansey: 368-434 (57.41 - 67.7%) -- guaranteed 2HKO
well now that's a bit more telling. most of the things that are run to get through pink blobs do not actually get through chansey - they only beat blissey. because of chansey's superior physical bulk, none of these attacks do enough damage to permanently cripple it or score a reliable 2hko. secret sword comes close because of stab though, but random tutored superpowers and psyshocks are not enough to beat chansey. they'll flatten blissey though.
now we'll take some veritably polarizing examples of things blobs should never be staying in on
252+ Atk Choice Band Haxorus Outrage vs. 4 HP / 252 Def Blissey: 700-825 (107.36 - 126.53%) -- guaranteed OHKO
252+ Atk Choice Band Haxorus Outrage vs. 0 HP / 252+ Def Eviolite Chansey: 468-552 (73.01 - 86.11%) -- guaranteed 2HKO
all these calcs should be an exceptionally clear demonstration of chansey's massive bulk advantage over blissey. there is a reason why chansey, not blissey, is a fixture of rain stall.
chansey and blissey are both a solid C rank.