i have played D&D since i pretty much started elementary school, and few systems could hold the same feel of Nostalgia. and well, D&D 1e and 2e had lots of commonly removaeble charts, 3.5 had lots of customizeability, and 4.0 had lots of hidden innovations. but my favorite Edition of D&D has got to be fifth, also called D&D Next/
Dungeons and Dragons Fifth Edition, your prestige classes are baked into the base class in a manner similar to Pathfinder's Archetypes. except that narrow field of Specialization is actually baked into your class, meaning you needn't be forced to trade the core of your class to be something new, and well, it makes designing subclasses easier. this perfectly marries the old 1st edition mechanic
second best part. when you multiclass as a spellcaster with either of 2 progressions, you only use 1 track of spell slots, based on your total character level and useable by all your spellcasting classes. meaning casters actually have less trouble multiclassing. meaning yeah, if you want a rogue 12 wizard 8, you will still have 9th level slots, but you will have a caster level of 12 for important things.
with the prime requisite system in the multiclassing section, which classes you can take are actually restricted by which scores you have a 13 or better in. this works well with the heroic NPC Array and well, actually helps weed out excessively crippled builds that make little in game sense, like fighters who did not gain strength or dexterity from a life of combat.
and the background system is amazing, and even allows for substitutions pulled from other backgrounds. meaning if your race or class already offer a skill or tool, you can trade that background skill or background tool for one you don't have. this also applies to racial skills, tools and weapons. for proficiencies at least. or hell, if there isn't an exact mixed background covered, you can literally mix and match to make one of your choosing. while this may seem like a cheap way to get perception or athletics on any class, with access to the proper desired tools and kits, the amount of customization is so amazing.
and well, characters feel much more competent at the low levels than they did in 3.5, without being as absurd as they were at the later levels. i mean, lower overall HP totals, but heightened access to out of combat healing, spammable cantrips, finesse weapons,and hell a decent amount of predetermined skills make 5e pretty amazing. plus, while you get fewer feats, each feat is a huge jump in specialization and there are even houserules for spending downtime to learn the use of skills and tools. and proficiency bonus is easy to calculate. and well, combat goes much smoother because there are fewer redundant spells and less overall numbers to crunch.
what can i actually say i dislike about my new favorite edition of D&D?
the lack of official published material is a problem for me. there aren't any pre made psionics rules and while homebrewing subclasses or subraces is easy enough, homebrewing a full base class or basic race is a different story
that wielding a weapon in both hands doesn't actually increase the amount of extra damage you deal from your strength, and well, in most cases, it is better to carry a shield than an offhand weapon or 2handed weapon.
that there is no bonus for a high intelligence for martial characters beyond saving throws.
i may sound like a 5th edition fangirl, but it is a freaking amazing system. nice, simple, customizeable and built with the best parts of its 4 ancestor systems. it has earned the name of "everybody's second favorite edition" due to having something good from everything.