I’m pretty late to the party here, but will post my opinion.
Non-legacy Mewtwo isn’t exactly worth “mewtwo or some other legacy legendary”.
Non-legacy Mewtwo isn’t particularly special (although at least it is best of type, just not a particularly useful type), so it’s worth the same as other legendaries that are either best of a not overly useful type, or legendaries that are close to best of a more useful type.
I would rank legendaries as follows (for lucky priority, for pure rating the legacies would be lower. I also ignore PvP below):
- Legacy Legendary that are very useful: Sky Attack Moltres and Shadow Ball Mewtwo
- Legendary that are best of type in a good type: Giratina-O, Kyogre, Groudon, Rayquaza, Raikou
- Legendary that are best of a weak type, or close to best of a good type: Non-legacy Mewtwo, Legacy TS Zapdos, Dialga, Palkia
- Legendary that aren’t utter garbage but also not above: Giratina-A, Entei, Lugia, Latios, Azelf
- Trash Legendary: Latias, Regis, Articuno, Suicine, Ho-oh, Heatran, Uxie, Mesprit
So if you get anything in tier 3 for Mewtwo it’s an even trade. If you get something in 1 or 2, it’s an extremely good trade for you. If you get something in 4, it’s not a terrible trade but is pretty bad (e.g. you should probably hope to get a higher level or shiny in return for it).
As for where these fall in line with the grand scheme of other potential trades?
- High level Rampardos
- High level Cranidos
- A: High level Shiny MM Metagross ; B: Shiny Lick Gengar (any level)
- High level Shiny SD Tyranitar
- Tier 1 legendaries above
- A: High level MM Metagross ; B: SD Tyranitar ; C: High level Lick or Shadow Claw Gengar
- A: High level Shiny Outrage Salamence ; B: High level Shiny FP Sceptile ; C: High level Shiny HC Swampert (not out yet) ; D: High level Shiny BB Blaziken ; E: High level shiny Mamoswine
- Tier 2 legendaries above
- Non-shiny variants of pokemon in 7 + low level lick/Shadow Claw Gengar + legacy GK Breloom
- High level top-tier attackers or their pre-evolutions if uncommon: e.g. Electivire and Roserade are both not particularly common for me
- Tier 3 legendaries above
- High level top-tier attackers or their pre-evolutions that are common: e.g. Machamp, Magnezone, Rhyperior, Weavile are all pretty common here.
If your trade partner can’t fill anything in those 12 tiers, I’d say remove the friend :P