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It seriously depends on your Magic/Drain scale...
The more the scale is "magic", the more the weak 1-path mages are good researchers... With a Drain scale you need better ones.
The national paths also play a role, with Air, Fire and Death having good research boosters.
AFAIK with limited -1/+1 scales the usual winning couple should be Sages (due to bonus) and Appr Theurg (sacred).With Drain scales Ulm Smiths are obvisouly fine, more so when given Fire lanterns.
Spectral mages are also quite good : no upkeep, access to death (Skull mentor).