Assuming you just want to constantly and reliably bomb a base and be ahead of your team and then die and repeat with minimal skill/attention requirement, here are best options for each nation I can think of.
US:
F-105D.
-Its rank 7 at 10.0, which can research top tier.
-Strike aircraft so you get a higher base bombing multiplier.
Carry 2 napalm bombs for 1 base and then head on anything you see and spam your vulcan, or use ballistic computer to quickly kill some ground targets after bombing. The point is to die as fast as possible and join the next match while trying for a kill or some ground pounding. Going to base and bombing again is not as time/reward efficient.
F-111F. Also works good, very fast but you will encounter more missiles that pose a treath early game, like the R-27ER that might damage you even when multipathing, or the phoenix from the F-14A
If you don’t care to grind Rank 8, use the F-104C instead with the same strategy, its better than the F-105 but you can’t research Rank 8 efficiently with it.
Germany:
Rank 7
-F-104G, same strategy as before. MiG-23 MLA also works but for pure passive bombing, the F-104 is better since its lower BR as just as fast, both are Rank 7 so you can research Rank 8.
USSR:
Rank 7
-Mig-23M at 11.0, its a worse MLA/MLD, but it’s also lower BR and still very fast.
Rank 6
-Su-7B/BKL, strike aircraft so also higher rewards for bombing.
Britain: (Britain kinda sucks for this, lack of very fast bombers and/or no napalm options)
Rank 7
-F-111C, is your best option. Bring 6x500lbs in 1 pylon and 1x2000lbs in the other. Its the least draggy option with the best multiplier for 1 base.
Japan:
Rank 6
-F-104J
Rank 7
-F-4EJ, it sucks but its the only viable option for it.
Because it sucks, you might consider using the F-16AJ instead at Rank 8 but you will often have to avoid phoenixes/fakours or other fox3 missiles and it’s not as braindead as playing lower tiers, and in downtiers you still get outpaced by Mig-23s and F-104s
China:
Rank 7
-F-104G it’s the worst F-104 for this because it is 10.7 with no countermeasures, but you still can reliably get to a base before anyone else. If you want to be faster than the other F-104s, carry just 2 napalm bombs and nothing else.
Italy:
Rank 6
-F-104G
Rank 7
-F-104S
-MiG-23MF
France:
Rank6
-F-84F if you get airspawn
Rank7
-F-104G
-F1C, very fast too, not as fast as the F-104 with napalms but you also get 2x magic2 for 1 or 2 brainead kills. Carry 2x2000lbs and a single 1000lbs and it’s just enough for a base.
-F-1C-200 is the premium option, very good grinder overall, same as the F1C.
Sweden:
Nothing… Absolutely crickets. Sorry. The only viable option is the Rank 5 Saab-105G/OE with airspawn.
Israel:
Rank 6
-A-4H, a very fast Skyhawk at 8.3, 2x napalm and a single 250lbs is enough for a base, also strike aircraft with higher rewards.
Rank 7 (not great options here)
-Nesher
-Kfir C.7, very fast but F-104s and MiG-23 will still outpace you, the kfir doesn’t have napalm and has lower rip speeds.
-Kfir canard is an ok premium option for this.
Tips:
-Carry just enough fuel to bomb and last for a couple minutes after.
-If you can’t reach bases in time before your teammates get them, consider lowering your payload, bombing 50 or 75% of a base is better than 0% and you also have higher reward multiplier up to 8 or 10 (strike aircraft) to compensate, so it’s not as bad.
-DO NOT go back to base to repeat, just try to get some kills or ground pound and die fast to join the next match.