The best way to play a Fighter was always dual-classing. Stepping out at 3rd (5th weapon proficiency), 7th (3/2 attacks), 9th (last d10+Con hit die before the fixed +3/level and 7th weapon proficiency), or 13th (2 attacks/round) gave you a very strong core for your character no matter what you went into. You would have the Fighter's advantages at low levels when they mattered most (high AC, higher max Con on your HD, percentile Strength), plus the extra attacks after you regained your abilities post-dual-classing to multiply whatever new abilities you got.
- Thieves netted the combat abilities to make them useful in combat and to turn backstabs truly deadly (percentile Str, weapon spec, extra attacks/round, all of which get backstab's multiplier), while also bringing out of combat utility (thieving skills) and even more hit points (+2 hp/level at 125k xp/level instead of +3 hp/level at 200k xp/level, and the potential a 10th hit die of 1d6+2 from Con).
- Wizards got the hp to not die to a strong gust of wind, the ability to wear Elven Chain (the one armor that doesn't interfere with spellcasting), and the ability to pick up a sword and deal some damage if the spells ran out. In 1st edition, Wizards also gained hit dice up to level 11 for potentially even more hp at the right breakpoints (Fighter 9/Wizard 11 with 18 Con has 9d10+2d4+40 hp, avg. 94.5, whereas a Fighter 11 would have 9d10+42 hp, avg. 91.5, at around the same xp total). Only up to level 10 in 2nd edition, though. Also, a Fighter/Wizard dual-class has a sick amount of hp with Tenser's Transformation.
- Clerics probably get the least benefit, because they're basically Fighters already. They get a bit more hp and extra attacks. Clerics are still held to their weapon restrictions, unlike Wizards and Thieves, so you don't even get that. Clerics alone also have the option to dual-class from Ranger instead, which puts a further damper on the Fighter/Cleric build, although Rangers don't bring much to the table, either, except in Icewind Dale's game engine where they weirdly gave full access to Druid spells as well with your Cleric casting.
- Druids get relieved of their weaker armor restrictions. Otherwise, see Cleric.