Intel Pentium II:
The fastest x86 processor. It also offers the best MMX and floating point
performance. Usually equipped with 512KB of level 2 cache. 
Intel Celeron: Basically a
Pentium II with either NO level 2 cache or with 128KB of level 2 cache. Steer
clear of those cacheless versions. A good performance at a low price if has the
cache. http://www.intel.com/products/desktop/processors/celeron/
On August 24, Intel announced the newest members of the Celeron family (perhaps better known by their code name, Mendocino), the 300A and the 333--both touting 128K of integrated L2 cache. The latter, as you'd expect, runs at 333 MHz. For the former, Intel added A to distinguish this chip from the preexisting 300-MHz cacheless Celeron.
ZDNet Products: "You might think that 128K of L2 cache couldn't possibly amount to much in terms of performance. But according to our benchmark tests, it does. Comparing Celeron computers with systems tested in our recent roundup of low-cost PCs. We found that the new 333-MHz Celeron can outperform a 300-MHz Pentium II. Yes, that's Pentium II."
http://www5.zdnet.com/products/stories/reviews/0,4161,2133134,00.html
Intel Pentium with MMX: Intel no
longer produces these. They offer good MMX and floating-point performance.
They are shipped in low-cost desktops and notebooks nowadays. http://www.intel.com/support/processors/pentiummmx/
AMD-K-6: AMD's fastest CPU,
although it's MMX and floating-point performance lags slightly behind that of Intel
processors. It's NT performance at a given clock speed is slightly better than that
of it's Intel competitors. http://www.amd.com/K6/k6docs/index.html
AMD-K6-2:
AMD's fastest CPU equipped with 3DNow technology (a set of new instructions
supported by MS's DirectX 6.0 that speeds 3D performance-currently limited to games, but
likely to be exploited by some business apps within a year). Also offers
improved MMX and floating-point performance over K6. 
Cyrix M II: This excels at
ordinary business programs, offering higher levels of performance on these
applications. Due partly to an efficient design, and partly to the on-chip unified
64KB of level 1 cache. But it runs at a lower clock speed than it's competitors, and
provides relatively poor performance on computationally intensive applications such as CAD
or imaging. The M II is also manufactured and sold by IBM as the IBM
6x86MX. 
IDT WinChip: An inexpensive chip
that offers decent performance on business applications, such as word processing or
spreadsheets, but relatively poor MMX and floating point performance.