I've been building solo-only characters since the F2P/P2P system was launched last year. I've tried just about everything in the interim and I've come to the conclusion that every build has weaknesses that make it unpalatable at some point in the process of leveling.
Melee builds that don't include at least a level of Monk and some Wisdom all wind up with AC too low to avoid random over-run deaths at some point, usually by level 10 or so. Builds that can't self-heal effectively are prone to devastating random assaults in which they lose a lot of hit points and then can't make the next shrine. A few crits in a row can produce the effect or just one failed Will save in which they're held for the critical seconds needed to produce 3 or 4 crits. Casters die very quickly when things go wrong and having a spell resisted or saved against at the wrong time usually means starting over, often when the end-boss is the mob who resisted.
The most solid builds I've tried have been Cleric/Monks and Paladin/Monk/Rogues. The self-healing is there, the saves are there, evasion is there, the Cleric/Monk even has the potential to morph into a true caster once the spells that make that worthwhile are available. Still, there are mobs and situations that begin to happen fairly early, as early as 8 or 9, where the builds are at a disadvantage and basically need to get lucky to get by the various challenges.
I've even tried the old mainstay of the Warforged Wizard, which is a bit weak until Firewall becomes available and then suffers greatly for a bunch of levels when it encounters fire immune packs. This build seems like the strongest of them all for soloing except against fire immune mobs and when it runs into vampires which it can't hope to kill due to their strong regeneration and it's fairly limited mana pool.
I've looked at the Exploiterr build and while it looks great at 20 it seems to have gaps of undesirability on the way there, like the long run with no evasion, and the fact that it is basically a wand-whipper for most of it's existence leading to heavy expenses early on in terms of gold for the wands. The Exploiterr also is a massive gear hound, looking for optimal gear at almost every level and needing top end game gear at 20 to do what it does.
The question I have is: what is the best solo build to go from 4 to 20 without the ability to massively twink the toon out and without buying pots from the store?