I play a fairly even mix. I usually don't play tier 1 characters, apart from Wizards here and there. On the list of characters I've made that actually ended up in games in the past year or two, including both 3.xe and 5e, I've got: 1 Rogue, 2 Daring Outlaw Rogue/Swashbucklers, 1 Barbarian, 1 Barbarian/Fighter, 1 Monk/Psywar, 3 Wizards, 1 Sorcerer, 1 Scout, 1 Ranger, 2 Warlocks, and 1 Warblade. I've also made but never played a couple Paladins, a Knight, a Druid, and an assortment of unfleshed stuff.
Of those... I can't honestly peg any of the 3.5e characters as having used nothing except core materials. Maybe the Wizard that was a generalist, but that still plucked some spells from non-core.
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That aside, we weren't talking about viability, we were talking about building a character the way we wanted to build them, i.e. in line with a certain motif or to fill a specific role. What I was getting at is that 3.ex does that by providing a mountain of splatbooks for piecemeal customization, where 5e does it by giving you some commonly-used archetypes which double as templates for making further ones to suit individual preferences.
Neither's bad, but they're certainly different, and each is better at certain things than the other.
e: GURPS comes with its own problems. It's further towards the end of the spectrum Neo was criticizing earlier in terms of making players work to make the system be what they need rather than having the options already there. The system's also more tuned for very general roleplay -- if a group is looking specifically for heroic fantasy, GURPS isn't the answer.