Hmm, well, while I can't recommend any lines to just free XP through, and my usual reply of stock grind and XP past aircraft you don't like have been taken, I will just make suggestions as aircraft you might want to save your free XP to bypass.
Fw 190 A-1: Very underwhelming aircraft, and is extremely painful to grind even when fully upgraded.
P-51A: Good aircraft in the performance department, however the anemic armament of only 4 .50 cals makes it extremely difficult to kill things.
I-210: Effectively a USSR P-51A, it's good in performance, however, armament is lacking in punch at only 2 20mm cannons, and they're ShVAK's.
Thus far, those are the only three aircraft I can strongly recommend bypassing entirely that I have used, however other suggestions friends have made would be:
Skipping to F4U-1 in USN Multi's: While serviceable and perfectly respectable aircraft, the F4U-1 sets the tone for the rest of the line and plays far different from the F4F.
Ju-87G/Hs 129 and Ju-88P: Some friends have recommended bypassing these due to them not being nearly as competitive as the Me 265 and you should play the Fw 189 to get some GA experience in.
Dh. 103 Hornet: Seen as the weakest tier 7 HF, can still do exceedingly well but some friends do not recommend it.
Bsh-2 and IL-2: Lack of a rear gunner is a pain for a GA.
Otherwise, just save for the stock grind, that'll be the real killer for many aircraft.